ANALYSIS OF SEQUENCES FLANKING THE VAP REGIONS OF DICHELOBACTER-NODOSUS - EVIDENCE FOR MULTIPLE INTEGRATION EVENTS, A KILLER SYSTEM, AND A NEW GENETIC ELEMENT

Dichelobacter nodosus is the causative agent of ovine footrot. The vap regions of the D. nodosus genome may have arisen by the integration o f a genetic element and may have a role in virulence. The virulent D. nodosus strain A198 has multiple copies of the vap regions. In the pre sent study, sequences to the left and right of vap regions 1, 2 and 3 of strain A198 were analysed by Southern blotting and DNA sequencing. The results suggest that vap regions 1 and 2 arose by independent inte gration events into different tRNA genes. The discovery of a second in tegrase gene (intB), a gene with similarity to bacteriophage repressor proteins (regA), and a gene similar to an ORF from a conjugative tran sposon (gepA). suggests that a second genetic element, either a bacter iophage or a conjugative transposon, is integrated next to vap region 3 in the D. nodosus genome. The arrangement of intB and the vap region s in three other virulent strains and one benign strain was determined using using Southern blotting and PCR. One strain, H1215, contained v apE' and not vapE, and thus resembles vap region 3, suggesting that va p region 3 also may have arisen by an independent integration event. I n all strains, a copy of lots was found next to the vap regions. The v ap regions contain two genes, vapA and toxA, with similarity to the hi g genes of the killer plasmid Rts1. Evidence is presented that vapA an d toxA have a similar function in D. nodosus.