The nucleotide sequence relatedness between the chromosome of and the virulence plasmid of was investigated using short DNA probes of < 2 kb covering the whole virulence plasmid sequence. Only one homologous region was detected. This region was subsequently cloned and partially sequenced. Sequences closely related to the gene and the ORFs and which are located downstream of the fimbrial pef operon of the virulence plasmid, were detected. Sequencing of the cloned DNA fragment also revealed identity with genes of the fimbrial operon characterized in the chromosome of These nucleotide sequences mapped upstream of the chromosomal region homologous to the virulence plasmid. The general organization of the cloned chromosomal fragment was similar to the fimbriae-encoding region of the virulence plasmid. The deduced product of in the virulence plasmid, as well as those of the corresponding ORFs in the homologous region of the chromosome and in the virulence plasmid (designated and , respectively), appeared to be related to the thioredoxin family of thiol:disulphide oxidoreductases. The gene was able to complement the DTT-sensitive phenotype, the inability to metabolize glucose 1-phosphate and the low alkaline phosphatase activity of a mutant of The gene partially complemented the lack of alkaline phosphatase activity, but not the other mutant phenotypes. The products of both genes could be detected using the T7 RNA polymerase promoter expression system. The estimated molecular masses of the products of the and genes by SDS-PAGE were 26 and 23 kDa, respectively, the first being in agreement with the deduced amino acid sequence and the latter, somewhat smaller. The processing of a possible leader peptide in the Dlp protein, but not in the Dlt protein, could be responsible for this difference. The Dlp protein appeared as a doublet band on SDS-PAGE, which is characteristic of the oxidized and reduced states of this kind of protein.


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