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.