Complete nucleotide sequence of the 27-kilobase virulence related locus (vrl) of Dichelobacter nodosus: Evidence for extrachromosomal origin

The vrl locus is preferentially associated with virulent isolates of the ov ine footrot pathogen, Dichelobacter nodosus, The complete nucleotide sequen ce of this 27.1-kb region has now been determined. The data reveal that the locus has a G+C content much higher than the rest of the D. nodosus chromo some and contains 22 open reading frames (ORFs) encoding products including a putative adenine-specific methylase, two potential DEAH ATP-dependent he licases, and two products with sequence similarity to a bacteriophage resis tance system. These ORFs are all in the same orientation, and most;are eith er overlapping or separated by only a few nucleotides, suggesting that they comprise an operon and are translationally coupled. Expression vector stud ies have led to the identification of proteins that correspond to many of t hese ORFs. These data, in combination with evidence of insertion of rrl int o the 3' end of an ssrA gene, are consistent with the hypothesis that the v rl locus was derived from the insertion of a bacteriophage or plasmid into the D. nososus genome.