Abstract
The
Campylobacter fetus
surface layer proteins (SLPs), encoded by five to nine
sapA
homologues, are major virulence factors. To characterize the
sapA
homologues further, a 65.9 kb
C. fetus
genomic region encompassing the
sap
locus from wild-type strain 23D was completely sequenced and analysed; 44 predicted open reading frames (ORFs) were recognized. The 53.8 kb
sap
locus contained eight complete and one partial
sapA
homologues, varying from 2769 to 3879 bp, sharing conserved 553–2622 bp 5′ regions, with partial sharing of 5′ and 3′ non-coding regions. All eight
sapA
homologues were expressed in
Escherichia coli
as antigenic proteins and reattached to the surface of SLP
−
strain 23B, indicating their conserved function. Analysis of the
sap
homologues indicated three phylogenetic groups. Promoter-specific polymerase chain reactions (PCRs) and
sapA
homologue-specific reverse transcription (RT)-PCRs showed that the unique
sapA
promoter can potentially express all eight
sapA
homologues. Reciprocal DNA recombination based on the 5′ conserved regions can involve each of the eight
sapA
homologues, with frequencies from 10
−1
to 10
−3
. Intragenic recombination between
sapA7
and
sapAp8
, mediated by their conserved regions with a 10
−1
–10
−2
frequency, allows the formation of new
sap
homologues. As divergent SLP C-termini possess multiple antigenic sites, their reciprocal recombination behind the unique
sap
promoter leads to continuing antigenic variation.