A MULTIPLE SITE-SPECIFIC DNA-INVERSION MODEL FOR THE CONTROL OF OMP1 PHASE AND ANTIGENIC VARIATION IN DICHELOBACTER-NODOSUS

The molecular cloning and sequence analysis of four structurally varia nt linked genes (omp1A,B,C,D) that encode the major outer membrane pro tein of Dichelobacter nodosus strain VCS1001 are described. The isolat ion of rearranged copies of omp1A and omp1B, and the identification in the 5' regions of all four genes of short cross-over-site sequences t hat were similar to the Din family of cross-over-site sequences, sugge sted that site-specific DNA inversion was involved in omp1 rearrangeme nt. Evidence for site-specific inversion of the 497 bp DNA fragment, w hich was located between the divergently orientated omp1A and omp1B ge nes, and which contained the promoter and 5' coding sequence of Omp1, was obtained by polymerase chain reaction-mediated amplification of in verted forms of these genes. However, to account for all of the omp1 g ene copies cloned in this study, a more widespread inversion phenomeno n must be involved in the rearrangement of these genes and a model for multiple site-specific DNA inversions at the omp1 locus is described. In this model the four structurally variant omp1 genes can be assembl ed from one of four structurally variant C-terminal coding regions and a conserved N-terminal coding region and can be expressed from a sing le promoter. It is postulated that this genetic capability endows D. n odosus with the ability to switch the antigenic specificity of one of its major surface proteins.