- Volume 150, Issue 10, 2004
Volume 150, Issue 10, 2004
- Microbiology Comment
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- Fungal Cell Wall Biogenesis: Building A Dynamic Interface With The Environment
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Multiple sequence signals determine the distribution of glycosylphosphatidylinositol proteins between the plasma membrane and cell wall in Saccharomyces cerevisiae
More LessGlycosylphosphatidylinositol (GPI)-anchored cell wall proteins (GPI-CWPs) play an important role in the structure and function of the cell wall in Saccharomyces cerevisiae and other fungi. While the majority of characterized fungal GPI-anchored proteins localize to the cell wall, a subset of GPI proteins are thought to reside at the plasma membrane and not to traffic significantly to the cell wall. The amino acids immediately upstream of the site of GPI anchor addition (the ω site) are the primary signal determining whether a GPI protein localizes to the cell wall or to the plasma membrane. Here, evidence was found that in addition to this ω-proximal signal, other sequences in the protein can impact the distribution of GPI proteins between cell wall and membrane. In particular, it was found that long regions rich in serine and threonine residues (a feature of many cell wall proteins) can override the ω-proximal signal and redirect a model GPI plasma membrane protein to the cell wall.
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Deficiencies in the essential Smp3 mannosyltransferase block glycosylphosphatidylinositol assembly and lead to defects in growth and cell wall biogenesis in Candida albicans
More LessGlycosylphosphatidylinositols (GPIs) are essential for viability in yeast and have key roles in cell wall construction. Assembly of Saccharomyces cerevisiae GPIs includes the addition of a fourth, side-branching mannose (Man) to the third Man of the core GPI glycan by the Smp3 mannosyltransferase. The SMP3 gene from the human pathogenic fungus Candida albicans has been cloned. CaSMP3 complements the inviable S. cerevisiae smp3 null mutant and, when expressed in an S. cerevisiae smp3/gpi13 double mutant, it permits in vivo conversion of the Man3-GPI precursor that accumulates in that mutant to a Man4-GPI. One allele of CaSMP3 was disrupted using the ura-blaster procedure, then the remaining allele was placed under the control of the glucose-repressible MAL2 promoter. Repression of CaSMP3 expression leads to accumulation of a GPI precursor glycolipid whose glycan headgroup contains three mannoses and bears a phosphodiester-linked substituent on its first Man. Under repressing conditions, cells exhibited morphological and cell wall defects and became inviable. CaSmp3p therefore adds a fourth, α1,2-linked Man to trimannosyl GPI precursors in C. albicans and is necessary for viability. Because addition of a fourth Man to GPIs is of less relative importance in mammals, Smp3p is a potential antifungal target.
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Systematic identification in silico of covalently bound cell wall proteins and analysis of protein–polysaccharide linkages of the human pathogen Candida glabrata
More LessCandida glabrata is an important cause of systemic candidiasis in humans. This paper reports a systematic analysis of the putative glycosylphosphatidylinositol-modified (GPI) proteins of C. glabrata, a large part of which are covalently bound to the cell wall glucan network and the remainder of which are retained in the plasma membrane, and of cell wall proteins (CWPs) which are covalently bound in a mild-alkali-sensitive manner. In silico genomic analysis revealed 106 putative GPI proteins. Fifty-one of these GPI proteins could be categorized as adhesive proteins, potentially implicated in fungus–host interactions or biofilm formation during the development of fungal infections. Eleven proteins belonged to well-known GPI protein families of glycoside hydrolases, probably involved in cell wall expansion and remodelling during growth. Other identified GPI proteins included phospholipases, aspartic proteases, homologues of ScEcm33p and ScKre1p, and structural CWPs. Interestingly, the GPI algorithm predicted three orthologues of an abundant CWP in S. cerevisiae, Cwp1p, which is absent in Candida albicans. To evaluate the in silico predictions, isolated cell walls were extracted using HF-pyridine, which specifically cleaves phosphodiester bonds, to release GPI-CWPs. Immunological analysis of the extract using one-dimensional SDS-PAGE and anti-ScCwp1p antiserum indicated the presence of a Cwp1p homologue in C. glabrata cell walls. Further analysis by two-dimensional gel electrophoresis and electrospray ionization tandem mass spectrometry (ESI-MS/MS) confirmed the presence of two of the predicted Cwp1p proteins, Cwp1.1p and Cwp1.2p. Crh1p, a putative 1,3-β-glucan remodelling enzyme, was also identified. In silico genomic analysis further revealed five putative Pir proteins (Pir1–5p) and five members of the Bgl2 glycoside hydrolase family 17, belonging to a class of putative CWPs that can be extracted with NaOH. Immunological analysis of mild-alkali-extracted CWPs showed the presence of a ScPir2p homologue. Together, these experimental data and in silico predictions represent the first systematic analysis of the C. glabrata cell wall proteome.
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Increased mortality of Saccharomyces cerevisiae cell wall protein mutants
More LessThe yeast cell wall contains an unusually high number of different mannoproteins. The physiological role of most of them is unknown and gene disruptions leading to depletion of different proteins do not affect major functions of the wall. In this work the phenotype of different single and multiple cell wall protein mutants was observed at the level of individual cells. It was found that the lack of the non-covalently bound wall proteins Scw4p, Scw10p and Bgl2p increases the mortality of Saccharomyces cerevisiae cells grown exponentially under standard laboratory conditions, as assayed by methylene blue staining. Mutation of SCW11, however, suppressed the phenotype of scw4scw10, or scw4scw10bgl2, indicating that Scw4p, Scw10p and Bgl2p act synergistically while Scw11p has an activity antagonistic to that of the other three proteins. Mutants lacking major covalently bound proteins, either all four described Pir-proteins or the five most abundant glycosylphosphatidylinositol (GPI)-anchored proteins (Ccw12p, Ccw13p/Dan1p, Ccw14p/Icwp1p, Tip1p and Cwp1p), also had increased mortalities, the first somewhat more and the latter less than that of scw4scw10bgl2. In all cases the observed phenotype was suppressed by the addition of an osmotic stabilizer to the growth medium, indicating that cells died due to decreased osmotic stability. If cells were grown to stationary phase, Scw-mutants showed only slightly increased mortality, but mutants lacking Pir- or GPI-anchored proteins had significantly increased sensitivity, suggesting that their physiological function is primarily expressed in stationary-phase cells. In many cases structures consisting of a living ccw5ccw6ccw7ccw8 (multiple Pir-protein mutant) mother with two methylene blue-stained daughters could be seen.
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Role of Pir1 in the construction of the Candida albicans cell wall
Searches in a Candida albicans database (http://genolist.pasteur.fr/CandidaDB/) identified two Individual Protein Files (IPF 15363 and 19968) whose deduced amino acid sequences showed 42 % and 45 % homology with Saccharomyces cerevisiae Pir4. The two DNA sequences are alleles of the same gene (CaPIR1) but IPF 19968 has a deletion of 117 bases. IPF 19968 encodes a putative polypeptide of 364 aa, which is highly O-glycosylated and has an N-mannosylated chain, four cysteine residues and seven repeats. Both alleles are expressed under different growth conditions and during wall construction by regenerating protoplasts. The heterozygous mutant cells are elongated, form clumps of several cells and are hypersensitive to drugs that affect cell wall assembly. CaPir1 was labelled with the V5 epitope and found linked to the 1,3-β-glucan of the C. albicans wall and also by disulphide bridges when expressed in S. cerevisiae.
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Rot1p of Saccharomyces cerevisiae is a putative membrane protein required for normal levels of the cell wall 1,6-β-glucan
Although ROT1 is essential for growth of Saccharomyces cerevisiae strain BY4741, the growth of a rot1Δ haploid was partially restored by the addition of 0·6 M sorbitol to the growth medium. Rot1p is predicted to contain 256 amino acids, to have a molecular mass of 29 kDa, and to possess a transmembrane domain near its C-terminus. Candida albicans and Schizosaccharomyces pombe have Rot1p homologues with high identity that also have predicted transmembrane domains. To explore the role of Rot1p, the phenotypes of the rot1Δ haploid were analysed. Deletion of ROT1 caused cell aggregation and an abnormal morphology. Analysis of the cell cycle showed that rot1Δ cells are delayed at the G2/M phase. The rot1Δ cells were resistant to K1 killer toxin and hypersensitive to SDS and hygromycin B, suggesting that they had cell wall defects. Indeed, greatly reduced levels of alkali-soluble and -insoluble 1,6-β-glucan, and increased levels of chitin and 1,3-β-glucan, were found in rot1Δ cells. Furthermore, the phenotypes of rot1Δ cells resemble those of disruption mutants of the KRE5 and BIG1 genes, which show greatly reduced levels of cell wall 1,6-β-glucan. Incorporation of glycosylphosphatidylinositol (GPI)-dependent cell wall proteins in big1Δ and rot1Δ cells was examined using a GFP–Flo1 fusion protein. GFP fluorescence was detected both on the cell surface and in the culture medium, suggesting that, in these mutants, mannoproteins may become only weakly bound to the cell wall and some of these proteins are released into the medium. Electron microscopic analyses of rot1Δ and big1Δ cells showed that the electron-dense mannoprotein rim staining was more diffuse and paler than that in the wild-type, and that the outer boundary of the cell wall was irregular. A big1Δrot1Δ double mutant had a growth rate similar to the corresponding single mutants, suggesting that Rot1p and Big1p have related functions in 1,6-β-glucan synthesis.
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Role of chitin synthase genes in Fusarium oxysporum
More LessThree structural chitin synthase genes, chs1, chs2 and chs3, were identified in the genome of Fusarium oxysporum f. sp. lycopersici, a soilborne pathogen causing vascular wilt disease in tomato plants. Based on amino acid identities with related fungal species, chs1, chs2 and chs3 encode structural chitin synthases (CSs) of class I, class II and class III, respectively. A gene (chs7) encoding a chaperone-like protein was identified by comparison of the deduced protein with Chs7p from Saccharomyces cerevisiae, an endoplasmic reticulum (ER) protein required for the export of ScChs3p (class IV) from the ER. So far no CS gene belonging to class IV has been isolated from F. oxysporum, although it probably contains more than one gene of this class, based on the genome data of the closely related species Fusarium graminearum. F. oxysporum chs1-, chs2- and chs7-deficient mutants were constructed through targeted gene disruption by homologous recombination. No compensatory mechanism seems to exist between the CS genes studied, since chitin content determination and expression analysis of the chs genes showed no differences between the disruption mutants and the wild-type strain. By fluorescence microscopy using Calcofluor white and DAPI staining, the wild-type strain and Δchs2 and Δchs7 mutants showed similar septation and even nuclear distribution, with each hyphal compartment containing only one nucleus, whereas the Δchs1 mutant showed compartments containing up to four nuclei. Pathogenicity assays on tomato plants indicated reduced virulence of Δchs2 and Δchs7 null mutants. Stress conditions affected normal development in Δchs2 but not in Δchs1 or Δchs7 disruptants, and the three chs-deficient mutants showed increased hyphal hydrophobicity compared to the wild-type strain when grown in sorbitol-containing medium. The chitin synthase mutants will be useful for elucidating cell wall biogenesis in F. oxysporum and the relationship between fungal cell wall integrity and pathogenicity.
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Interspore bridges: a new feature of the Saccharomyces cerevisiae spore wall
More LessThe Saccharomyces cerevisiae spore wall is a multilaminar coat that surrounds individual spores and protects them from environmental insult. Scanning electron microscopy reveals that the four spores of an ascus are connected by interspore bridges. Transmission electron microscopy of spores indicates that these bridges are continuous with the outer layers of the spore wall. In chs3 mutants, which lack the chitosan and dityrosine layers of the spore wall, bridges are absent. By contrast, in dit1 mutants, which lack only the dityrosine layer, bridges are present, suggesting that the bridges may be composed of chitosan. Interspore bridges are shown to be necessary to hold spores together after release from the ascus. A function for these bridges in the maintenance of heterozygous markers in a homothallic yeast population is proposed.
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Scw10p, a cell-wall glucanase/transglucosidase important for cell-wall stability in Saccharomyces cerevisiae
More LessGlycosyl hydrolases and transferases are crucial for the formation of a rigid but at the same time plastic cell wall in yeasts and fungi. The Saccharomyces cerevisiae glucan hydrolase family 17 (GH17) contains the soluble cell-wall proteins Scw4p, Scw10p, Scw11p and Bgl2p. For Bgl2p, endoglucanase/glucanosyltransferase activity has been demonstrated, and Scw11p has been shown to be involved in cell separation. Here, Scw4p and Scw10p, which show 63 % amino acid identity, were characterized. scw4 and scw10 single mutants were sensitive towards cell-wall destabilizing agents, suggesting a role in cell-wall assembly or maintenance. Simultaneous deletion of SCW4 and SCW10 showed a synergistic effect, and activated the cell-wall compensatory mechanism in a PKC1-dependent manner. Both the amount of cell-wall chitin and the amount of mannoproteins attached to chitin were increased in mutant scw4scw10. Deletion of CHS3 proved the critical role of chitin in scw4scw10. However, the mannoprotein Sed1p and the glucan synthase Fks2p were also crucial for cell-wall stability in mutant scw4scw10. The exchange of two conserved glutamate residues localized in the putative catalytic domain of GH17 family members strongly suggests that Scw10p acts as a 1,3-β-glucanase or as a 1,3-β-glucanosyltransferase. In addition, the synthetic interactions between Bgl2p and Scw10p which support a functional cooperation in cell-wall assembly were analysed. The data suggest that Scw4p and Scw10p act as glucanases or transglucosidases in concert with other cell-wall proteins to assure cell-wall integrity.
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Yeast Kre1p is GPI-anchored and involved in both cell wall assembly and architecture
More LessKre1p is a cell surface O-glycoprotein involved in a late stage of 1,6-β-glucan formation in the yeast Saccharomyces cerevisiae. Disruption of KRE1 leads to a 40 % reduction in the overall 1,6-β-glucan content of the cell wall. This paper shows that in a yeast Δkre1 null mutant, neither an N-terminal-truncated Kre1p nor Kre1p variants lacking a C-terminal glycosylphospatidylinositol (GPI) attachment site are capable of achieving normal function in glucan assembly, while full-length Kre1p completely complements a Δkre1 null mutation and restores cell wall 1,6-β-glucan content up to wild-type level. In a yeast gpi1 mutant, a green-fluorescent-protein-tagged Kre1p derivative is secreted into the medium, indicating an at least transient GPI-anchoring stage of Kre1p during its processing within the yeast secretory pathway. In contrast to the severe defect in cell wall β-d-glucan, the amount of cell wall mannoproteins is not significantly decreased in a Δkre1 disruptant, as could be confirmed in competition assays by investigating toxin binding to isolated cell wall mannoproteins. Since the yeast Δkre1 mutant differed in its sensitivity to zygocin and K28, two killer viral protein toxins that use different cell wall mannoprotein populations as a primary toxin receptor, it can be concluded that in a yeast Δkre1 background, mannoproteins do not differ significantly in total amount from a Kre1+ wild-type but rather in their expression and distribution at the cell surface. Taken together, these data favour and suggest a structural, rather than enzymic, function of Kre1p in yeast cell wall assembly.
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The ER-Golgi v-SNARE Bet1p is required for cross-linking α-agglutinin to the cell wall in yeast
More LessIn Saccharomyces cerevisiae, glycosylphosphatidylinositol (GPI)-anchored cell wall mannoproteins, including α-agglutinin, are secreted to the cell surface through vesicular transport pathways. At the cell surface the GPI anchors are cleaved within the glycan, then transglycosylated to form a covalent cross-link to 1,6-β-glucan. Among mutants that were temperature-sensitive for growth and for ability to cross-link the mannoprotein α-agglutinin to the cell wall, one strain was complemented by BET1, which encodes an ER-Golgi v-SNARE. Temperature-sensitive mutations in BET1 caused aberrations in cell wall structure, including excretion of α-agglutinin into the medium, sensitivity to lysis with Zymolyase and hypersensitivity to Calcofluor White. At restrictive temperatures, bet1 mutations block secretion of invertase and other proteins, but α-agglutinin was excreted into the extracellular medium. In wild-type parental or bet1 cells, secretion of α-agglutinin also continued after protein synthesis was blocked with cycloheximide. This secretion was due to continued export of a significant amount of α-agglutinin from compartments distal to the BET1-dependent secretion step. Thus, in bet1 cells the ER-Golgi block allowed secretion to continue, but prevented cell wall incorporation of the α-agglutinin. Therefore, a mutation early in the secretion pathway caused aberrant cell wall synthesis by preventing localization of key components required in wall cross-links.
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Saccharomyces cerevisiae Bni4p directs the formation of the chitin ring and also participates in the correct assembly of the septum structure
More LessIn Saccharomyces cerevisiae cytokinesis is efficiently achieved when a concerted series of events take place at the neck region, leading to septum formation. Here it is shown that Bni4p plays a crucial role in this process. Δbni4 mutants contain normal amounts of chitin and show normal chitin synthase III (CSIII) activity, but are partially resistant to Calcofluor White (CFW), probably due to the striking pattern of chitin distribution. CFW vital staining shows that chitin is synthesized in daughter cells and that it is also asymmetrically deposited at the mother-side of the neck in large-budded cells. This specific pattern coincides with that of Chs4p and Chs3p proteins. Alternatively, staining of unbudded cultures confirmed that Bni4p directs early chitin ring assembly, but is no longer required for the chitin deposition that occurs late in the cell cycle at cytokinesis. Consequently, this work provides a strategy to genetically discriminate between the absence of chitin synthesis (Δchs3 mutant) and failure in chitin ring assembly (Δbni4 mutants). The characterization of double mutants affected in chitin synthesis and primary septum (PS) assembly (Δmyo1 and Δchs2) provides evidence for the cooperation of Bni4p in PS formation besides its role in chitin ring assembly. In addition, it is shown that the chitin ring, but not the late deposition of chitin, cooperates in the correct assembly of the actomyosin ring and the PS when the biological function of the septins is compromised. We conclude that Bni4p is not only required for the assembly of the chitin ring, but is also involved in septum architecture and the maintenance of neck integrity.
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Candida albicans mutants in the BNI4 gene have reduced cell-wall chitin and alterations in morphogenesis
More LessThe Candida albicans BNI4 gene was identified by homology to the Saccharomyces cerevisiae orthologue and encodes a predicted 1655 amino acid protein. In S. cerevisiae most cell-wall chitin is associated with primary septum formation and Bni4p is involved in tethering the Chs3p chitin synthase enzyme to the mother-bud neck by forming a bridge between a regulatory protein Chs4p and the septin Cdc10p. CaBni4p shows 20 % overall identity to the ScBni4p, with 73 % identity over the C-terminal 63 amino acids, which includes a putative protein phosphatase type 1 (PP1) binding domain. Northern blot analysis revealed a transcript of the expected size that was expressed in both yeast and hyphal growth forms. C. albicans has more chitin in its cell wall than S. cerevisiae, and again most chitin is synthesized by CaChs3p. The function of CaBNI4 was investigated by performing a targeted gene disruption using the ‘Ura-blaster’ method to delete amino acids 1120–1611 that are essential for function. The resulting Cabni4Δ/Cabni4Δ null mutants formed lemon-shaped yeast cells and had a 30 % reduction in cell-wall chitin, reduced hyphal formation on solid serum-containing medium and increased sensitivity to SDS and increased resistance to Calcofluor White. The Cabni4Δ/Cabni4Δ null mutants were unaffected in chitin ring formation, but often exhibited displaced bud sites with more obvious but flattened birth scars. Therefore, unlike in S. cerevisiae, the Cabni4 mutant apparently alters chitin distribution throughout the cell wall and not exclusively at the bud-neck region.
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Survival and cytokinesis of Saccharomyces cerevisiae in the absence of chitin
More LessMost fungal cell walls are constructed with significant amounts of chitin, a linear polysaccharide that contributes mechanical resistance to the structure. In the yeast Saccharomyces cerevisiae, chitin is synthesized by three different isozymes, each of which has a separate cellular function. In this yeast, the most important role of chitin is in cytokinesis, when a thin primary septum is synthesized by chitin synthase II to separate mother and daughter cells. If no primary septum can be formed, an irregular remedial septum is synthesized, a process that relies on chitin synthase III. It was found that, with osmotic stabilization, S. cerevisiae tolerates a loss of all chitin synthase activities. Chitin-deficient mutants display a cytokinesis defect which leads to the formation of cell chains with incompletely separated cytoplasms. In these mutants septa are formed rarely. The few septa found are bulky structures which contain inclusions of cytoplasm. Nuclear division proceeds under these conditions, demonstrating that there is no cell cycle arrest triggered by a failure to form a septum between mother and daughter cell. A genetic suppressor arises quickly in chitin-deficient mutants, giving rise to the synthesis of chitin-free remedial septa. The suppressed chitin-free mutants grow well without osmotic stabilization and display hyper-resistance against the chitin-synthase inhibitor polyoxin D.
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The Aspergillus fumigatus cell wall is organized in domains that are remodelled during polarity establishment
Aspergillus fumigatus is a life-threatening and increasingly frequent pathogen of the immunocompromised. Like other filamentous fungi A. fumigatus grows in a highly polar manner, adding new cell wall to the apical region of hyphae. mAbs were raised against isolated A. fumigatus cell walls. Fifteen antibodies bound reproducibly to isolated cell walls in ELISAs and to the walls of intact cells in immunofluorescence experiments. Surprisingly, individual mAbs showed distinct patterns of localization. Six antibodies labelled exclusively conidial or basal regions, seven labelled apical regions and a single antibody labelled both basal and apical regions of hyphae. Ten antibodies did not label the walls adjacent to septa. In double labelling experiments with representative mAbs there was little or no overlap between epitopes recognized. These labelling patterns suggest that the wall is made up of basal and apical domains that differ in composition or organization and that the wall region flanking septa differs from other regions of the lateral wall. In time-course experiments of early A. fumigatus growth, mAb16C4 failed to label isotropically expanding cells and labelled emerging germ tubes and branches. The same mAb failed to label the Aspergillus nidulans swoC mutant, which is defective in polarity establishment. However, mAb16C4 did label the A. nidulans swoA mutant, which completes polarity establishment, but is defective in polarity maintenance. Thus, mAb16C4 appears to recognize a cell wall change that occurs during polarity establishment. In immunogold labelling and transmission electron microscopy (TEM) experiments, conidia, basal regions and apical regions of thin-sectioned cells labelled with mAb16C4. That only apical regions labelled in intact cells (immunofluorescence) while conidial, basal and apical regions labelled in thin-sectioned cells (TEM) suggests that the 16C4 epitope is present along the whole hypha, but is masked everywhere except the tip until polarity establishment. That is to say, some remodelling of the wall during polarity establishment exposes the 16C4 epitope. The 16C4 epitope was partially purified from A. fumigatus total protein by passage through hydrophobic interaction and anion-exchange columns. The resulting single ELISA-positive fraction showed relatively few bands by SDS-PAGE and silver staining and a strong band by Western blotting with the16C4 mAb. Sequencing of the fraction yielded a predicted peptide with a 6-amino acid exact match to a region of the Cat1 protein previously identified as an immunodominant A. fumigatus catalase that localizes to the cell wall and is secreted to the medium. Experiments are under way to determine if mAb16C4 recognizes Cat1 or another protein that co-purifies with Cat1.
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CRR1, a gene encoding a putative transglycosidase, is required for proper spore wall assembly in Saccharomyces cerevisiae
In Saccharomyces cerevisiae, sporulation is a developmental process that converts a single cell into four haploid spores. The four haploid nuclei are encapsulated within multilayered spore walls that protect them against stressful conditions. The formation of the spore-specific cell wall is a highly coordinated process that requires the participation of enzymic activities for biosynthesis, degradation, and cross-linking between components. Here the sporulation-specific gene CRR1, encoding a putative transglycosidase that is required for proper spore wall assembly, is described. Both the transcription of CRR1 and the synthesis of Crr1p were induced biphasically under sporulating conditions, with a first expression peak displaying kinetics similar to those of the middle to middle-late sporulation-specific genes, and a second late peak after 24 h under these conditions. Localization studies revealed that Crr1p localized to the spore wall that surrounds each of the four ascospores within the mature asci. Mutation of this gene had no effect on the efficiency of spore formation. However, crr1 mutant spores were sensitive to hydrolytic enzymes such as glusulase and to heat-shock treatments, underscoring the importance of this gene in the proper formation and assembly of the ascospore wall. Moreover, the deletion of CRR1 had additive effects with respect to the sensitivity of cda1 cda2 mutants to these treatments. Interestingly, overexpression of CRR1 not only complemented the phenotype of the crr1 strain but also rendered spores more resistant to the stress conditions than the wild-type. Like other mutants impaired in the formation of the spore outer layer, crr1 mutants were permeable to Calcofluor White. Finally, detailed analysis by electron microscopy of the spore walls in the crr1 mutants revealed a defect in the assembly of the spore wall components, suggesting a role for Crr1p in the cross-linking between the inner (glucan/mannoprotein) and the outer (chitosan/dityrosine) spore layers.
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Mutational analysis of the cytoplasmic domain of the Wsc1 cell wall stress sensor
More LessWsc1 is a member of a family of highly O-glycosylated cell surface proteins that reside in the plasma membrane of Saccharomyces cerevisiae and function as sensors of cell wall stress. These proteins activate the cell wall integrity signalling pathway by stimulating the small G-protein Rho1, protein kinase C (Pkc1) and a MAP kinase cascade. The cytoplasmic domains of Wsc1 family members interact with the Rom2 guanine nucleotide exchange factor to stimulate GTP-binding of Rho1. Here, a mutational analysis of the cytoplasmic domain of Wsc1 is presented. The data identify two regions of the Wsc1 cytoplasmic tail that are conserved with other family members as important for Rom2 interaction. These regions are separated by an inhibitory region, which includes a cluster of seryl residues that appear to be phosphorylated. Mutational analysis of these residues supports a model in which Wsc1 interaction with Rom2 is negatively regulated by phosphorylation.
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Differential roles of PDK1- and PDK2-phosphorylation sites in the yeast AGC kinases Ypk1, Pkc1 and Sch9
More LessSaccharomyces cerevisiae Pkh1 and Pkh2 (orthologues of mammalian protein kinase, PDK1) are functionally redundant. These kinases activate three AGC family kinases involved in the maintenance of cell wall integrity: Ypk1 and Ypk2, two closely related, functionally redundant enzymes (orthologues of mammalian protein kinase SGK), and Pkc1 (orthologue of mammalian protein kinase PRK2). Pkh1 and Pkh2 activate Ypk1, Ypk2 and Pkc1 by phosphorylating a Thr in a conserved sequence motif (PDK1 site) within the activation loop of these proteins. A fourth protein kinase involved in growth control and stress response, Sch9 (orthologue of mammalian protein kinase c-Akt/PKB), also carries the conserved activation loop motif. Like other AGC family kinases, Ypk1, Ypk2, Pkc1 and Sch9 also carry a second conserved sequence motif situated in a region C-terminal to the catalytic domain, called the hydrophobic motif (PDK2 site). Currently, there is still controversy surrounding the identity of the enzyme responsible for phosphorylating this second site and the necessity for phosphorylation at this site for in vivo function. Here, genetic and biochemical methods have been used to investigate the physiological consequences of phosphorylation at the PDK1 and PDK2 sites of Ypk1, Pkc1 and Sch9. It was found that phosphorylation at the PDK1 site in the activation loop is indispensable for the essential functions of all three kinases in vivo, whereas phosphorylation at the PDK2 motif plays a non-essential and much more subtle role in modulating the ability of these kinases to regulate the downstream processes in which they participate.
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Studies on the regulation of the two-component histidine kinase gene CHK1 in Candida albicans using the heterologous lacZ reporter gene
More LessThe two-component histidine kinase Chk1p of Candida albicans has been implicated in the regulation of cell wall biosynthesis. Deletion of CHK1 results in avirulence that in part may be due to the increased sensitivity of mutant strains to polymorphonuclear leukocytes. The mutant also does not adhere to human oesophageal tissue in vitro, probably as a consequence of its altered cell wall. In the current study, a CHK1 promoter-lacZ reporter (CHK1prlacZ) construct was expressed in wild-type C. albicans strain CAI4 and in two-component signal transduction mutants to determine the effect of environmental stress conditions on the regulation of CHK1 and the co-regulatory activities among these proteins. It is shown that lacZ expression varied according to the type of growth conditions and incubation time; expression was also influenced by the strain background. lacZ expression in CAI4 was greater at 37 °C and at a pH of 3·5 and in the presence of 4 mM H2O2, 0·1 mM menadione, 10 % serum or 1·5 M NaCl compared to cells grown at 30 or 42 °C. The increases in expression were time-dependent and not observed until cells were incubated for 120 min in these conditions (P<0·05). As a correlate of the increase in transcription of CHK1-lacZ in the presence of H2O2, the chk1 mutant was more sensitive than wild-type and revertant cells to H2O2 in vitro. In addition to strain CAI4, we also measured CHK1p-lacZ reporter activity of mutants deleted in genes encoding other two-component proteins such as the response regulator gene SSK1, the histidine kinases, SLN1 and NIK1, and the HOG1 MAP kinase. Of these proteins, Ssk1p and Sln1p are presumed to mediate phosphotransfer to the HOG1 [hyperosmotic glycerol] MAP kinase pathway during oxidative and perhaps osmotic stress in C. albicans. Compared to strain CAI4, lacZ reporter activity increased significantly in the ssk1 mutant under all growth conditions after a 10 and 120 min incubation (P<0·0001). lacZ expression in the ssk1 mutant was less at 42 °C compared to all other growth conditions (P<0·05). Furthermore, lacZ reporter activity also increased in the hog1 mutant of C. albicans. These data suggest that SSK1 and HOG1 indirectly or directly negatively regulate CHK1 under most growth conditions tested. In the sln1 mutant, downregulation of CHK1 was observed in all growth conditions compared to strain CAI4 (P<0·05), while regulation of lacZ in the nik1 mutant was similar to strain CAI4 except when cells were incubated in the presence of 4 mM H2O2 for 120 min (P<0·05). Western blot analysis was used to determine the role of Chk1p in phosphorylation of Hog1p under oxidative or osmotic stress. It was found that Hog1p was phosphorylated in the chk1 mutant similar to wild-type CAF2-1 cells, although the temporal events of phosphorylation differed slightly in mutant cells. These results show that transcription of CHK1, as measured by the lacZ reporter assay, is statistically increased when cells are exposed to several types of stress or when incubated in 10 % serum in a mutant-specific background and at a specific time point. Of importance, our data also suggest that lacZ expression is indirectly or directly regulated by the HOG1 MAP kinase pathway, although a determination of its position in this pathway or in a cross-talking pathway awaits additional studies.
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The cell wall stress response in Aspergillus niger involves increased expression of the glutamine : fructose-6-phosphate amidotransferase-encoding gene (gfaA) and increased deposition of chitin in the cell wall
Perturbation of cell wall synthesis in Saccharomyces cerevisiae, either by mutations in cell wall synthesis-related genes or by adding compounds that interfere with normal cell wall assembly, triggers a compensatory response to ensure cell wall integrity. This response includes an increase in chitin levels in the cell wall. Here it is shown that Aspergillus niger also responds to cell wall stress by increasing chitin levels. The increased chitin level in the cell wall was accompanied by increased transcription of gfaA, encoding the glutamine : fructose-6-phosphate amidotransferase enzyme, which is responsible for the first and a rate-limiting step in chitin synthesis. Cloning and disruption of the gfaA gene in A. niger showed that it was an essential gene, but that addition of glucosamine to the growth medium could rescue the deletion strain. When the plant-pathogenic fungus Fusarium oxysporum and food spoilage fungus Penicillium chrysogenum were subjected to cell wall stress, the transcript level of their gfa gene increased as well. These observations suggest that cell wall stress in fungi may generally lead to activation of the chitin biosynthetic pathway.
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Impaired PRPP-synthesizing capacity compromises cell integrity signalling in Saccharomyces cerevisiae
More LessIn Saccharomyces cerevisiae, PRS genes comprise a family of five paralogous genes. Previously, it has been shown that in the cell the gene products are organized into two interacting complexes, one of which is a heterodimer and the other a heterotrimer. Here, it has been demonstrated that in addition to supplying the cell with the key metabolic intermediate PRPP [5-phospho-d-ribosyl-1(α)-pyrophosphate], the gene products contribute to the maintenance of cell integrity. Specifically, the phosphorylation of Rlm1, one of the end points of the cell integrity signalling pathway, is significantly impaired following deletion of any one of the PRS genes, in particular PRS1 and PRS3. This is reflected in changes in the expression of the alternative 1,3-β-glucan synthase catalytic subunit, Fks2, as measured by its promoter activity. Yeast two-hybrid analysis has shown that Prs1, specifically the non-homologous region, NHR1-1 and Prs3, and to a lesser extent Prs2 and Prs4, interact with the MAPK (mitogen-activated protein kinase) of the cell integrity pathway, Slt2. When PRS1 is lacking, the basal level of phosphorylation of Slt2 is increased. Furthermore, prs1Δ and prs3Δ strains have an increased chitin content under normal growth conditions. α-Factor sensitivity and Calcofluor White resistance associated with the lack of Prs1 and Prs3 corroborate the involvement of these two gene products in cell integrity signalling. It is postulated that Prs polypeptides play a significant role in the remodelling of the cell wall and may have a direct involvement in cell integrity signalling.
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The GPI-anchored protein CaEcm33p is required for cell wall integrity, morphogenesis and virulence in Candida albicans
More LessEcm33p is a widely distributed fungal protein with functional relevance, clearly demonstrated by ecm33Δ mutant phenotypes, mainly related to the cell wall. Homology searches with Saccharomyces cerevisiae genes identified Candida albicans Ecm33p, as well as the two other proteins of its family: Pst1p and the product of YCL048w. C. albicans Ecm33p is a 423 aa protein which has the typical features of cell-surface GPI proteins and is able to complement S. cerevisiae ecm33Δ cell wall defects. Heterozygous (RML1) and homozygous (RML2) mutants of CaECM33 were obtained, as well as a single and a double reintegrant (RML3 and RML4, respectively). Caecm33 mutant strains displayed an aberrant morphology, being more rounded and bigger than the wild-type, suggesting morphogenetic defects. They also exhibited cell wall defects, with enhanced sensitivity to different compounds that interfere in polymerization of cell wall components (Calcofluor white, Congo red and hygromycin B) and a marked tendency to flocculate extensively. In addition, CaEcm33p is required for normal C. albicans yeast-to-hyphae transition in vitro. In liquid medium (5 % serum), the transition was delayed in Caecm33 mutants, and after 24 h the culture contained very abnormal large and rounded cells. On solid medium (10 % serum, Spider or SLADH) RML2 failed to produce hyphae and media invasiveness. CaECM33 showed a gene dosage effect, demonstrated by the intermediate phenotype of the heterozygous mutants RML1 and confirmed by Northern blot analysis. Furthermore, CaEcm33p is also involved in C. albicans virulence. In a murine systemic model of infection, 100 % mouse survival and no kidney or brain colonization were obtained 30 days after infection with 106 Candida cells of any homozygous or heterozygous Caecm33Δ mutant tested. In contrast, all mice infected with parental or RML4 (two CaECM33 copies reintegrated) strains died in a few days, showing that, in these conditions, two CaECM33 copies were required for virulence.
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Ectophosphatase activity in conidial forms of Fonsecaea pedrosoi is modulated by exogenous phosphate and influences fungal adhesion to mammalian cells
A cell-wall-associated phosphatase in hyphae of Fonsecaea pedrosoi, a fungal pathogen causing chromoblastomycosis, was previously characterized by the authors. In the present work, the expression of an acidic ectophosphatase activity in F. pedrosoi conidial forms was investigated. The surface phosphatase activity in F. pedrosoi is associated with the cell wall, as demonstrated by transmission electron microscopy. This enzyme activity was strongly inhibited by exogenous inorganic phosphate (Pi). Accordingly, removal of Pi from the culture medium of F. pedrosoi resulted in a marked (130-fold) increase of ectophosphatase activity. With the artificial phosphatase substrate p-nitrophenyl phosphate, a K m value of 0·63±0·04 mM was estimated for the phosphatase activity of fungal cells strongly expressing the enyzme activity. This enzyme activity was not modulated by cations. Conidia with greater ectophosphatase activity showed greater adherence to mammalian cells than did fungi cultivated in the presence of Pi (low phosphatase activity). Surface phosphatase activity was apparently involved in the adhesion to host cells, since the enhanced attachment of F. pedrosoi to host cells was reversed by pre-treatment of conidia with phosphatase inhibitor. Since conidial forms are the putative infectious propagules in chromoblastomycosis, the expression and activity of acidic surface phosphatases in these cells may contribute to the early mechanisms required for disease establishment.
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- Biochemistry And Molecular Biology
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Stress induces depletion of Cdc25p and decreases the cAMP producing capability in Saccharomyces cerevisiae
More LessIn Saccharomyces cerevisiae the cAMP-dependent protein kinase A pathway antagonizes the cellular response to stress. It is shown here that the cellular content of Cdc25p, the upstream activator of Ras and adenylyl cyclase, decays upon various stresses such as heat shock and oxidative and ethanol shocks, whereas its phosphorylation level and its localization are unaffected. In parallel with the reduction of Cdc25p, the maximal capacity of the cell to accumulate cAMP decreases when its feedback regulation is abolished. A deletion of CDC25 prevents this decrease. Paradoxically, in wild-type cells, with normal feedback regulation, the level of cAMP, which is much lower, is not reduced but is rather increased upon stress. These observations are consistent with a role of Cdc25p in sensing and transducing stress to downstream targets, either through a cAMP-independent pathway or by large fluctuations in the cAMP content of the cell.
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Amino acid residues involved in cold adaptation of isocitrate lyase from a psychrophilic bacterium, Colwellia maris
More LessTo investigate the mechanism of cold adaptation of isocitrate lyase (ICL; EC 4.1.3.1) from the psychrophilic bacterium Colwellia maris, Gln207 and Gln217 of this enzyme were substituted by His and Lys, respectively, by site-directed mutagenesis. His184 and Lys194 of ICL from Escherichia coli, corresponding to the two Gln residues of C. maris ICL, are highly conserved in the ICLs of many organisms and are known to be essential for catalytic function. The mutated ICLs (Cm-Q207H and Cm-Q217K, respectively) and wild-type enzymes of C. maris and E. coli (Cm-WT and Ec-WT) with His-tagged peptides were overexpressed in E. coli cells and purified to homogeneity. Thermolabile Cm-WT and mutated ICLs were susceptible to digestion with trypsin, while relatively thermostable Ec-WT was resistant to trypsin digestion, suggesting that the thermostability and resistance to tryptic digestion of the ICLs are related. Cm-Q207H and Cm-Q217K showed specific activities similar to Cm-WT at temperatures between 30 °C and 40 °C, but their activities between 10 °C and 25 °C were decreased, indicating that the two Gln residues of the C. maris ICL play important roles in its cold adaptation. Phylogenetic analysis of ICLs from various organisms revealed that the C. maris ICL can be categorized in a novel group, subfamily 3, together with several eubacterial ICLs.
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The role of polyhydroxyalkanoate biosynthesis by Pseudomonas aeruginosa in rhamnolipid and alginate production as well as stress tolerance and biofilm formation
More LessPseudomonas aeruginosa is capable of synthesizing polyhydroxyalkanoic acids (PHAs) and rhamnolipids, both of which are composed of 3-hydroxydecanoic acids connected by ester bonds, as well as synthesizing the biofilm matrix polymer alginate. In order to study the influence of PHA biosynthesis on rhamnolipid and alginate biosynthesis, as well as stress tolerance and biofilm formation, isogenic knock-out mutants deficient in PHA biosynthesis were generated for P. aeruginosa PAO1 and the alginate-overproducing P. aeruginosa FRD1. A gentamicin-resistance cassette was inserted replacing the 3′ region of phaC1, the whole of phaZ and the 5′ region of phaC2. Gas chromatography/mass spectrometry analysis showed that PHA accumulation was completely abolished in both strains. Interestingly, this gene replacement did not abolish rhamnolipid production. Thus, as previously suggested, the PHA synthase is not directly involved in rhamnolipid biosynthesis. In the PHA-negative mutant of mucoid FRD1 alginate biosynthesis was not affected, whereas in the PHA-negative PAO1 mutant an almost threefold increase in biosynthesis was observed compared to the wild-type. Consistently, PHA accumulation in FRD1 contributed only 4·7 % of cell dry weight, which is fourfold less than in PAO1. These data suggest that PHA biosynthesis and alginate biosynthesis are in competition with respect to a common precursor. The surface attachment and biofilm development of the PHA-negative mutants were also compared to those of wild-type strains in glass flow-cell reactors. PHA-negative mutants of P. aeruginosa PAO1 and FRD1 showed reduced attachment to glass. However, the PAO1 PHA-negative mutant, in contrast to the wild-type, formed a stable biofilm with large, distinct and differentiated microcolonies characteristic of alginate-overproducing strains of P. aeruginosa. The stress tolerance of PHA-negative mutants with respect to elevated temperature was strongly impaired. These data indicated a functional role for PHA in stress response and tolerance.
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Membrane topology and mutational analysis of Escherichia coli CydDC, an ABC-type cysteine exporter required for cytochrome assembly
More LessCytochrome bd is a respiratory quinol oxidase in Escherichia coli. Besides the structural genes (cydA and cydB) encoding the oxidase complex, the cydD and cydC genes, encoding an ABC-type transporter, are required for assembly of this oxidase. Recently, cysteine has been identified as a substrate (allocrite) that is transported from the cytoplasm by CydDC, but the mechanism of cysteine export to the periplasm and its role there remain unknown. To initiate an understanding of structure–function relationships in CydDC, its membrane topography was analysed by generating protein fusions between random and selected residues in the two polypeptides with both alkaline phosphatase and β-galactosidase. CydD and CydC are experimentally shown each to have six transmembrane segments, two major cytoplasmic loops and three minor periplasmic loops; both termini of each protein face the cytoplasm. The cydD1 allele is shown to have two point mutations (G319D, G429E) within the ATP-binding domain of CydD; either mutation alone is sufficient to cause loss or severe reduction of cytochrome bd assembly. A comparative sequence analysis prompted the targeting of residues in CydD for site-directed mutational analysis, which identified (i) the ‘start’ methionine residue, (ii) essential residues in the ATP-binding site (Walker sequence A) and (iii) a duplicated positively charged heptameric motif, R-G/T-L/M-X-T/V-L-R, in CydD cytoplasmic loop II. The replacement of arginines in these motifs with glycines resulted in Cyd− phenotypes; however, activity could be restored at these positions by replacing the glycine with lysine or histidine and hence returning the positive charge. The conservation of these charges in CydD-like proteins indicates functional importance. Evolutionary aspects of bacterial cyd genes are discussed.
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GerE-independent expression of cotH leads to CotC accumulation in the mother cell compartment during Bacillus subtilis sporulation
Evidence is presented that expression of the cotH gene, whose product is required for the correct assembly of the Bacillus subtilis spore coat, is negatively controlled by the transcriptional regulator GerE. Mutations in the GerE-box, present in the cotH promoter region, increased expression of this gene, which also remained elevated during late stages of sporulation, when in wild-type cells cotH is normally turned off. Such alterations of cotH expression did not significantly affect spore coat structure or function but caused the accumulation of CotC molecules in the mother cell compartment, most likely as a consequence of CotH-mediated protection of CotC.
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Detoxification of hydrogen peroxide and expression of catalase genes in Rhodobacter
More LessThe two related facultatively photosynthetic bacteria Rhodobacter sphaeroides and Rhodobacter capsulatus show different sensitivities against peroxide stress. R. sphaeroides is able to tolerate higher concentrations of H2O2 and exhibits higher catalase activity than R. capsulatus. The katE gene of R. sphaeroides and the katG gene of R. capsulatus are strongly induced by H2O2. This induction depends on the presence of the OxyR protein, which is able to bind to the promoter regions of these genes. In addition to katE R. sphaeroides harbours the katC gene, which shows no significant response to H2O2 but is induced in stationary phase.
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Crotonyl-coenzyme A reductase provides methylmalonyl-CoA precursors for monensin biosynthesis by Streptomyces cinnamonensis in an oil-based extended fermentation
More LessIt is demonstrated that crotonyl-CoA reductase (CCR) plays a significant role in providing methylmalonyl-CoA for monensin biosynthesis in oil-based 10-day fermentations of Streptomyces cinnamonensis. Under these conditions S. cinnamonensis L1, a derivative of a high-titre producing industrial strain C730.1 in which ccr has been insertionally inactivated, produces only 15 % of the monensin yield. Labelling of the coenzyme A pools using [3H]-β-alanine and analysis of intracellular acyl-CoAs in the L1 and C730.1 strains demonstrated that loss of ccr led to lower levels of the monensin precursor methymalonyl-CoA, relative to coenzyme A. Expression of a heterologous ccr gene from Streptomyces collinus fully restored monensin production to the L1 mutant. Using C730.1 and an oil-based extended fermentation an exceptionally efficient and comparably intact incorporation of ethyl [3,4-13C2]acetoacetate into both the ethylmalonyl-CoA- and methylmalonyl-CoA-derived positions of monensin was observed. No labelling of the malonyl-CoA-derived positions was observed. The opposite result was observed when the incorporation study was carried out with the L1 strain, demonstrating that ccr insertional inactivation has led to a reversal of carbon flux from an acetoacetyl-CoA intermediate. These results dramatically contrast similar analyses of the L1 mutant in glucose-soybean medium which indicate a role in providing ethylmalonyl-CoA but not methylmalonyl-CoA, thus causing a change in the ratio of monensin A and monensin B analogues, but not the overall monensin titre. These results demonstrate that the relative contributions of different pathways and enzymes to providing polyketide precursors are thus dependent upon the fermentation conditions. Furthermore, the generally accepted pathways for providing methylmalonyl-CoA for polyketide production may not be significant for the S. cinnamonensis high-titre monensin producer in oil-based extended fermentations. An alternative pathway, leading from the fatty acid catabolite acetyl-CoA, via the CCR-catalysed reaction is proposed.
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Regulation of exopolysaccharide synthesis in Rhizobium sp. strain TAL1145 involves an alternative sigma factor gene, rpoH2
More LessExopolysaccharide (EPS) produced by Rhizobium sp. strain TAL1145 has been shown to be essential for effective nodulation on Leucaena leucocephala (leucaena). This paper reports the isolation and characterization of an alternative sigma factor gene, rpoH2, involved in the regulation of EPS synthesis in TAL1145. Disruption of this gene in TAL1145 resulted in a Calcofluor-dim mutant RUH102 that produced approximately 18 % of the amount of EPS made by TAL1145. This mutation did not affect the normal growth of RUH102 in free-living state. RUH102 induced few nitrogen-fixing nodules, resulting in a significant reduction in total nitrogen content in leucaena. It was complemented for EPS production and nodulation by a 2·0 kb HindIII fragment of TAL1145. Sequence analysis of this fragment revealed the rpoH2 ORF of 870 bp that encoded a protein of 32 kDa. Expression of the rpoH2 ORF in Escherichia coli also revealed a 32 kDa protein. A PCR-constructed clone of 1263 bp, containing the rpoH2 ORF and its upstream putative regulatory region, complemented RUH102 for EPS defects. Comparison of the RpoH2 sequence to proteins in the databases showed significant similarity to RpoH-like sigma factors of other Gram-negative bacteria. By constructing several exo : : Tn3Hogus fusions and transferring them to the backgrounds of TAL1145 and RUH102, it was demonstrated that RpoH2 positively regulates the transcription of some exo genes.
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- Environmental Microbiology
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Microbial community structure in a thermophilic anaerobic hybrid reactor degrading terephthalate
A thermophilic terephthalate-degrading methanogenic consortium was successfully enriched for 272 days in an anaerobic hybrid reactor, and the microbial structure was characterized using terminal RFLPs, clone libraries and fluorescence in-situ hybridization with rRNA-targeted oligonucleotide probes. All the results suggested that Methanothrix thermophila-related methanogens, Desulfotomaculum-related bacterial populations in the Gram-positive low-G+C group, and OP5-related populations were the key members responsible for terephthalate degradation under thermophilic methanogenic conditions except during periods when the reactor experienced heat shock and pump failure. These perturbations caused a significant shift in bacterial population structure in sludge samples taken from the sludge bed but not from the surface of the packing materials. After system recovery, many other bacterial populations emerged, which belonged mainly to the Gram-positive low-G+C group and Cytophaga–Flexibacter–Bacteroides, as well as β-Proteobacteria, Planctomycetes and Nitrospira. These newly emerged populations were probably also capable of degrading terephthalate in the hybrid system, but were out-competed by those bacterial populations before perturbations.
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- Genes And Genomes
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Comparative genomics using Candida albicans DNA microarrays reveals absence and divergence of virulence-associated genes in Candida dubliniensis
More LessCandida dubliniensis is a pathogenic yeast species closely related to Candida albicans. However, it is less frequently associated with human disease and displays reduced virulence in animal models of infection. Here comparative genomic hybridization was used in order to assess why C. dubliniensis is apparently less virulent than C. albicans. In these experiments the genomes of the two species were compared by co-hybridizing C. albicans microarrays with fluorescently labelled C. albicans and C. dubliniensis genomic DNA. C. dubliniensis genomic DNA was found to hybridize reproducibly to 95·6 % of C. albicans gene-specific sequences, indicating a significant degree of nucleotide sequence homology (>60 %) in these sequences. The remaining 4·4 % of sequences (representing 247 genes) gave C. albicans/C. dubliniensis normalized fluorescent signal ratios that indicated significant sequence divergence (<60 % homology) or absence in C. dubliniensis. Sequence divergence was identified in several genes (confirmed by Southern blot analysis and sequence analysis of PCR products) with putative virulence functions, including the gene encoding the hypha-specific human transglutaminase substrate Hwp1p. Poor hybridization of C. dubliniensis genomic DNA to the array sequences for the secreted aspartyl proteinase-encoding gene SAP5 also led to the finding that SAP5 was absent in C. dubliniensis and that this species possesses only one gene homologous to SAP4 and SAP6 of C. albicans. In addition, divergence and absence of sequences in several gene families was identified, including a family of HYR1-like GPI-anchored proteins, a family of genes homologous to a putative transcriptional activator (CTA2) and several ALS genes. This study has confirmed the close relatedness of C. albicans and C. dubliniensis and has identified a subset of unique C. albicans genes that may contribute to the increased prevalence and virulence of this species.
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Urease activity of enterohaemorrhagic Escherichia coli depends on a specific one-base substitution in ureD
More LessThe authors previously reported that most enterohaemorrhagic Escherichia coli (EHEC) strains do not express urease activity, despite having the urease gene. This study compared the nucleotide sequences of the urease gene clusters of a urease-activity-positive and a urease-activity-negative strain. The results showed that in the urease-activity-negative strain, ureD, a gene encoding a chaperone protein, had a single base substitution that encoded a premature stop codon resulting in a short ORF. The premature stop codon in ureD was commonly found in urease-activity-negative EHEC strains, but not in urease-activity-positive strains. Urease activity was detected after complementing the urease-activity-negative strain with ureD from the urease-activity-positive strain. Furthermore, introduction of the urease gene cluster from the urease-activity-negative strain into an amber suppressor phenotype Escherichia coli strain, DH5α, conferred the ability to produce the active urease. These results suggest that the lack of urease activity in most EHEC strains is due to a premature stop codon in ureD.
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Nucleotide sequences and comparison of two large conjugative plasmids from different Campylobacter species
More LessTwo large tetracycline resistance (TcR) plasmids have been completely sequenced, the pTet plasmid (45·2 kb) from Campylobacter jejuni strain 81-176 and a plasmid pCC31 (44·7 kb) from Campylobacter coli strain CC31 that was isolated from a human case of severe gastroenteritis in the UK. Both plasmids are mosaic in structure, having homologues of genes found in a variety of different commensal and pathogenic bacteria, but nevertheless, showed striking similarities in DNA sequence and overall gene organization. Several predicted proteins encoded by genes involved in conjugation showed highest homology to proteins found in Actinobacillus actinomycetemcomitans, a periodontal pathogen. In addition to replication- and conjugation-associated genes, both plasmids carried a tet(O) gene encoding tetracycline resistance, a 6 kb ORF encoding a putative methylase and a number of genes of unknown function. The pTet plasmid co-exists in C. jejuni strain 81-176 with a smaller, previously characterized, non-conjugative plasmid pVir that also encodes a type IV secretion system (T4SS) that may affect virulence. In contrast, the T4SS encoded by pTet and pCC31 are shown to mediate bacterial conjugation between Campylobacter. The possible origin and evolution of pCC31 and pTet is discussed.
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- Pathogens And Pathogenicity
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Mycoplasmas associated with canine infectious respiratory disease
Canine infectious respiratory disease (CIRD) is a complex infection that occurs worldwide predominantly in kennelled dogs, and several bacterial and viral micro-organisms have been associated with outbreaks of CIRD. However, few studies have comprehensively examined the species of mycoplasma present in healthy dogs and those with CIRD. As part of an extensive study investigating the micro-organisms involved in CIRD, the species of mycoplasma present throughout the respiratory tract of dogs with and without CIRD were determined. Mycoplasmas were cultured from tonsillar, tracheal and bronchial lavage samples, and identified to the species level by PCR and sequencing. Mycoplasma cynos was demonstrated on the ciliated tracheal epithelium by in situ hybridization and was the only mollicute found to be associated with CIRD, but only in the lower respiratory tract. Isolation of M. cynos was correlated with an increased severity of CIRD, younger age and a longer time in the kennel.
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The Mycobacterium tuberculosis Rv1099c gene encodes a GlpX-like class II fructose 1,6-bisphosphatase
More LessThere are now abundant data indicating that Mycobacterium tuberculosis uses fatty acids as a carbon source in vivo. A key enzyme in gluconeogenesis, missing in the original annotation of the M. tuberculosis genome, is fructose 1,6-bisphosphatase (FBPase; EC 3.1.3.11). The authors have shown that M. tuberculosis Rv1099c, a glpX homologue, can complement Escherichia coli mutants lacking FBPase. The protein encoded by Rv1099c was shown to have FBPase activity. Rv1099c was expressed at significant levels in M. tuberculosis, and may encode the major FBPase of this pathogen.
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