DELINEATION OF THE VIRULENCE-RELATED LOCUS (VRL) OF DICHELOBACTER-NODOSUS

Dichelobacter nodosus is the primary pathogen implicated in ovine footrot. In this paper we have delineated a 27 kb locus, termed the virulence-related locus (vrl), that was essentially specific for virulent D. nodosus isolates. The precise ends of this locus were mapped and the sequences of the junction regions from the virulent strain A198 were compared to corresponding sequences from the benign isolate C305. The left end of the vrl locus was located in a sequence similar to that of the small stable 10Sa RNA molecule of Escherichia coli, next to a phage-attachment-site-like sequence, which indicated that the vrl locus might have arisen by the integration of a phage. However, no attachment-like sequence could be found at the right end of the vrl locus. In the chromosome of the benign strain the sequences bordering vrl were not contiguous but were separated by about 3 kb. It was concluded that the divergence of the benign and virulent strains at this locus was a multi-step process. Several potential ORFs were identified at the junction regions but only one ORF, encoding a 126 kDa protein, was expressed in a T7 expression system in E. coli.