Campylobacter fetus sap inversion occurs in the absence of recA function

Phase variation of Campylobacter fetus surface layer proteins (SLPs) occurs by inversion of a 6.2-kb DNA segment containing the unique sap promoter, p ermitting expression of a single SLP-encoding gene. Previous work has shown that the C. fetus sap inversion system is RecA dependent. When we challeng ed a pregnant ewe with a recA mutant of wild-type C. fetus (strain 97-211) that expressed the 97-kDa SLP, 15 of the 16 ovine-passaged isolates express ed the 97-kDa protein, However, one strain (97-209) expressed a 127-kDa SLP , suggesting that chromosomal rearrangement may have occurred to enable SLP switching. Lack of RecA function in strains 97-211 and 97-209 was confirme d by their sensitivity to the DNA-damaging agent methyl methane-sulfonate, Southern hybridization and PCR of these strains indicated that the aphA ins ertion into recA was stably present. However, Southern hybridizations demon strated that in strain 97-209 inversion had occurred in the sap locus. PCR data confirmed inversion of the 6,2-kb DNA element and indicated that in th ese recA mutants the sap inversion frequency is reduced by 2 to 3 log(10) u nits compared to that in the wild type. Thus, although the major sap invers ion pathway in C. fetus is RecA dependent, alternative lower-frequency, Rec A-independent inversion mechanisms exist.