Structure and genotypic plasticity of the Campylobacter fetus sap locus

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.