NESTED DNA INVERSION OF CAMPYLOBACTER-FETUS S-LAYER GENES IS RECA DEPENDENT

Wild-type strains of Campylobacter fetus are covered by a monomolecula r array of surface layer proteins (SLPs) critical for virulence. Each cell possesses eight SLP gene cassettes, tightly clustered in the geno me, that encode SLPs of 97 to 149 kDa. Variation of SLP expression occ urs by a mechanism of nested DNA rearrangement that involves the inver sion of a 6.2-kb sapA promoter-containing element alone or together wi th one or more flanking SLP gene cassettes. The presence of extensive regions of identity flanking the 5' and 3' ends of each SLP gene casse tte and of a Chi-like recognition sequence within the 5' region of ide ntity suggests that rearrangement of SLP gene cassettes may occur by a generalized (RecA-dependent) homologous recombination pathway. To exp lore this possibility, we cloned C. fetus recA and created mutant stra ins by marker rescue, in which recA is disrupted in either S+ or S- st rains. These mutants then were assessed for their abilities to alter S LP expression either in the presence or absence of a complementary shu ttle plasmid harboring native recA. In contrast to all previously repo rted programmed DNA inversion systems, inversion in C. fetus is recA d ependent.