Abstract
Phase variation of
Campylobacter fetus
surface layer proteins (SLPs) occurs by inversion of a 6.2-kb DNA segment containing the unique
sap
promoter, permitting expression of a single SLP-encoding gene. Previous work has shown that the
C. fetus sap
inversion system is RecA dependent. When we challenged 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 expressed 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 confirmed by their sensitivity to the DNA-damaging agent methyl methanesulfonate. Southern hybridization and PCR of these strains indicated that the
aphA
insertion into
recA
was stably present. However, Southern hybridizations demonstrated 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 these
recA
mutants the
sap
inversion frequency is reduced by 2 to 3 log
10
units compared to that in the wild type. Thus, although the major
sap
inversion pathway in
C. fetus
is RecA dependent, alternative lower-frequency, RecA-independent inversion mechanisms exist.