Ovine footrot is a debilitating and highly infectious disease that is primarily caused by the Gram-negative, anaerobic bacterium Dichelobacter nodosus. The major antigens implicated in virulence are the type IV fimbriae and extracellular proteases. The fimbriae show sequence and structural similarity to other type IV fimbriae, this similarity extends to genes that are involved in fimbrial biogenesis. Several acidic and basic extracellular serine proteases are produced by both virulent and benign isolates of D. nodosus. Subtle functional differences in these proteases appear to be important in virulence. In addition, there are two chromosomal regions that have a genotypic association with virulence. The partially duplicated and rearranged vap regions appear to have arisen from the insertion of a plasmid into a tRNA gene via an integrase-mediated site-specific insertion event. The 27 kb vrl region has several genes often found on bacteriophages and has inserted into an ssrA gene that may have a regulatory role in the cell. The determination of the precise role that each of these genes and gene regions has in virulence awaits the development of methods for the genetic analysis and manipulation of D. nodosus.

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The recA gene from Clostridium perfringens was cloned using degenerate oligonucleotide primers designed from conserved regions of RecA proteins from other bacteria. The 1089 bp gene encoded a putative RecA protein with 69% amino acid sequence similarity to the RecA protein from Bacillus subtilis. The C. perfringens recA gene was induced by exposure to methyl methanesulphonate and complemented a recA mutant of Escherichia coli. A Cheo box was identified in the region upstream of the gene. Since this SOS-like operator site is conserved in many DNA-damage-inducible recA gene regions from Gram-positive bacteria, the results suggest that the regulation of the C. perfringens recA gene also involves the binding of a LexA-like protein to this site.

The causative agent of ovine footrot, the gram-negative anaerobeDichelobacter nodosus, produces polar type IV fimbriae, which are the major protective antigens. The D. nodosusgenes fimN, fimO, and fimP are homologs of the Pseudomonas aeruginosa fimbrial assembly genes, pilB, pilC, and pilD, respectively. Both the pilD and fimP genes encode prepilin peptidases that are responsible for cleavage of the leader sequence from the immature fimbrial subunit. To investigate the functional similarity of the fimbrial biogenesis systems from these organisms, the D. nodosus genes were introduced intoP. aeruginosa strains carrying mutations in the homologous genes. Analysis of the resultant derivatives showed that thefimP gene complemented a pilD mutant ofP. aeruginosa for both fimbrial assembly and protein secretion. However, the fimN and fimO genes did not complement pilB or pilC mutants, respectively. These results suggest that although the PilD prepilin peptidase can be functionally replaced by the heterologous FimP protein, the function of the PilB and PilC proteins may require binding or catalytic domains specific for the P. aeruginosafimbrial assembly system. The transcriptional organization and regulation of the fimNOP gene region were also examined. The results of reverse transcriptase PCR and primer extension analysis suggested that these genes form an operon transcribed from two ς70-type promoters located upstream of ORFM, an open reading frame proximal to fimN. Transcription of theD. nodosus fimbrial subunit was found to increase in cells grown on solid media, and it was postulated that this regulatory effect may be of significance in the infected footrot lesion.

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