Streptomyces
Over the last century, Streptomyces bacteria – and their metabolic products – have revolutionized modern medicine. These little pharmaceutical factories produce a vast array of natural products that have been co-opted for medical and agricultural therapies. In addition to their metabolic sophistication, Streptomyces also exhibit remarkable developmental and regulatory complexity.
Guest-edited by Dr Marie Elliot, this collection of keynote research articles will highlight fascinating aspects of Streptomyces biology, and the advances that are providing us with newfound insight and appreciation for these extraordinary bacteria.
Collection Contents
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Phylogenetic analyses of antibiotic-producing Streptomyces sp. isolates obtained from the stingless-bee Tetragonisca angustula (Apidae: Meliponini)
Many insects have been associated with actinobacteria in protective symbiosis where antimicrobial metabolites inhibit host pathogens. However, the microbiota of neotropical insects such as the stingless-bee Tetragonisca angustula is poorly explored. T. angustula is a meliponid bee widely distributed in Latin America, its honey is traditionally exploited because of its ethno-pharmacological properties and its antimicrobial activity has been demonstrated. Also, the well-structured nest of this species allows exploration of the microbiota of its different components. Even though Streptomyces spp. have been cultured from stingless-bees, little is known about their role in this insect–microbe relationship. In this study, we examined the association between culturable actinobacteria and T. angustula, and evaluated the isolates’ potential as antimicrobial producers. We isolated 51 actinobacteria from adult bees and different substrates of the hive of T. angustula (pollen and honey storage, garbage pellets and cerumen). We then performed a 16S rRNA phylogenetic analysis that clusters the bacteria to previously described lineages of host-associated Streptomyces . In addition, all the isolates were classified according to their antibacterial activity against human pathogens, measured by a growth inhibition test based on diffusion in agar. More than 50 % of our isolates exhibit antimicrobial activity, mainly to Gram-positive bacteria and fungi and only two against Gram-negative bacteria. Additionally, we obtained electron micrographs of adult bees with what appears to be patches of hyphae with Streptomyces -like cell morphology on their body surface. Our results suggest that T. angustula possibly uptakes and transfers actinobacteria from the environment, acting as vectors for these potentially beneficial organisms. This research provides new insights regarding the microbiota associated with T. angustula and justify future studies exploring the full diversity of the microbial community associated with the hive and the possible exchange of microbes with the crops they pollinate.
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A phylogenetic and evolutionary analysis of antimycin biosynthesis
More LessStreptomyces species and other Actinobacteria are ubiquitous in diverse environments worldwide and are the source of, or inspiration for, the majority of antibiotics. The genomic era has enhanced biosynthetic understanding of these valuable chemical entities and has also provided a window into the diversity and distribution of natural product biosynthetic gene clusters. Antimycin is an inhibitor of mitochondrial cytochrome c reductase and more recently was shown to inhibit Bcl-2/Bcl-XL-related anti-apoptotic proteins commonly overproduced by cancerous cells. Here we identify 73 putative antimycin biosynthetic gene clusters (BGCs) in publicly available genome sequences of Actinobacteria and classify them based on the presence or absence of cluster-situated genes antP and antQ, which encode a kynureninase and a phosphopantetheinyl transferase (PPTase), respectively. The majority of BGCs possess either both antP and antQ (L-form) or neither (S-form), while a minority of them lack either antP or antQ (IQ- or IP-form, respectively). We also evaluate the biogeographical distribution and phylogenetic relationships of antimycin producers and BGCs. We show that antimycin BGCs occur on five of the seven continents and are frequently isolated from plants and other higher organisms. We also provide evidence for two distinct phylogenetic clades of antimycin producers and gene clusters, which delineate S-form from L- and I-form BGCs. Finally, our findings suggest that the ancestral antimycin producer harboured an L-form gene cluster which was primarily propagated by vertical transmission and subsequently diversified into S-, IQ- and IP-form biosynthetic pathways.
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Phosphate and oxygen limitation induce respiratory nitrate reductase 3 synthesis in stationary-phase mycelium of Streptomyces coelicolor A3(2)
More LessThe saprophytic actinobacterium Streptomyces coelicolor A3(2) requires oxygen for filamentous growth. Surprisingly, the bacterium also synthesizes three active respiratory nitrate reductases (Nar), which are believed to contribute to survival, or general fitness, of the bacterium in soil when oxygen becomes limiting. In this study, we analysed Nar3 and showed that activity of the enzyme is restricted to stationary-phase mycelium of S. coelicolor. Phosphate limitation was shown to be necessary for induction of enzyme synthesis. Nar3 synthesis was inhibited by inclusion of 20 mM phosphate in a defined ‘switch assay’ in which highly dispersed mycelium from exponentially growing cultures was shifted to neutral MOPS-glucose buffer to induce Nar3 synthesis and activity. Quantitative assessment of nar3 transcripts revealed a 30-fold induction of gene expression in stationary-phase mycelium. Transcript levels in stationary-phase mycelium incubated with phosphate were reduced by a little more than twofold, suggesting that the negative influence of phosphate on Nar3 synthesis was mainly at the post-transcriptional level. Furthermore, it was demonstrated that oxygen limitation was necessary to induce high levels of Nar3 activity. However, an abrupt shift from aerobic to anaerobic conditions prevented appearance of Nar3 activity. This suggests that the bacterium regulates Nar3 synthesis in response to the energy status of the mycelium. Nitrate had little impact on regulation of the Nar3 level. Together, these data identify Nar3 as a stationary-phase nitrate reductase in S. coelicolor and demonstrate that enzyme synthesis is induced in response to both phosphate limitation and hypoxia.
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Posttranslational processing of the xylanase Xys1L from Streptomyces halstedii JM8 is carried out by secreted serine proteases
The xylanase Xys1L from Streptomyces halstedii JM8 is known to be processed extracellularly, to produce a protein of 33·7 kDa, Xys1S, that retains catalytic activity but not its cellulose-binding capacity. This paper demonstrates that at least five serine proteases isolated from Streptomyces spp. have the ability to process the xylanase Xys1L. The genes of two of these extracellular serine proteases, denominated SpB and SpC, were cloned from Streptomyces lividans 66 (a strain commonly used as a host for protein secretion), sequenced, and overexpressed in S. lividans; both purified proteases were able to process Xys1L in vitro. Three other previously reported purified Streptomyces serine proteases, SAM-P20, SAM-P26 and SAM-P45, also processed Xys1L in vitro. The involvement of serine proteases in xylanase processing-degradation in vivo was demonstrated by co-expression of the xylanase gene (xysA) and the gene encoding the serine protease inhibitor (SLPI) from S. lividans. Co-expression prevented processing and degradation of Xys1L and resulted in a threefold increase in the xylanase activity present in the culture supernatant. SpB and SpC also have the capacity to process other secreted proteins such as p40, a cellulose-binding protein from S. halstedii JM8, but do not have any clear effect on other secreted proteins such as amylase (Amy) from Streptomyces griseus and xylanase Xyl30 from Streptomyces avermitilis.
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Physical mapping shows that the unstable oxytetracycline gene cluster of Streptomyces rimosus lies close to one end of the linear chromosome
More LessA restriction map of the 8 Mb linear chromosome of Streptomyces rimosus R6-501 was constructed for the enzymes AseI (13 fragments) and DraI (7 fragments). Linking clones for all 12 AseI sites and 5 of the 6 DraI sites were isolated. The chromosome has terminal inverted repeats of 550 kb, which are the longest yet reported for a Streptomyces species. The oxytetracycline gene cluster lies about 600 kb from one end, which might account for its frequent spontaneous amplification and deletion. Several other markers were localized on the chromosome (dnaA and recA, the rrn operons, the attachment site for pSAM2 and prophages RP2 and RP3). Comparison of the conserved markers with the map of Streptomyces coelicolor A3(2) suggested there are differences in genome organization between the two species.
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