Abstract
The virulence of
Campylobacter fetus
, a bacterial pathogen of ungulates and humans, is mediated in part by the presence of a paracrystalline surface layer (S-layer) that confers serum resistance. The subunits of the S-layer are S-layer proteins (SLPs) that are secreted in the absence of an N-terminal signal sequence and attach to either type A or B
C. fetus
lipopolysaccharide in a serospecific manner. Antigenic variation of multiple SLPs (encoded by
sapA
homologs) of type A strain 23D occurs by inversion of a promoter-containing DNA element flanked by two
sapA
homologs. Cloning and sequencing of the entire 6.2-kb invertible region from
C. fetus
23D revealed a probable 5.6-kb operon of four overlapping genes (
sapCDEF
, with sizes of 1,035, 1,752, 1,284, and 1,302 bp, respectively) transcribed in the opposite direction from
sapA
. The four genes also were present in the invertible region of type B strain 84-107 and were virtually identical to their counterparts in the type A strain. Although SapC had no database homologies, SapD, SapE, and SapF had predicted amino acid homologies with type I protein secretion systems (typified by
Escherichia coli
HlyBD/TolC or
Erwinia chrysanthemi
PrtDEF) that utilize C-terminal secretion signals to mediate the secretion of hemolysins, leukotoxins, or proteases from other bacterial species. Analysis of the C termini of four
C. fetus
SLPs revealed conserved structures that are potential secretion signals. A
C. fetus sapD
mutant neither produced nor secreted SLPs.
E. coli
expressing
C. fetus sapA
and
sapCDEF
secreted SapA, indicating that the
sapCDEF
genes are sufficient for SLP secretion.
C. fetus
SLPs therefore are transported to the cell surface by a type I secretion system.