SEGMENTAL CONSERVATION OF SAPA SEQUENCES IN TYPE-B CAMPYLOBACTER-FETUS CELLS

Campylobacter fetus cells may exist as either of two defined serogroup s (type A or B) based on their lipopolysaccharide (LPS)composition. Wi ld-type strains contain surface array proteins (S-layer proteins) that have partial antigenic cross-reactivity but bind exclusively to LPS f rom homologous (type A or B) cells. Type A cells possess 8 homologs of sapA, which encodes a 97-kDa S-layer protein; the gene products of th ese homologs have a conserved N terminus of 184 amino acids. To furthe r explore the structural relationships between the C. fetus S-layer pr oteins and their encoding genes, we sought to clone and express an S-l ayer protein from type B strain 84-91, The cloned type B gene (sapB) w as similar in structure to the previously cloned type A gene (sapA) an d encoded a full-length 936-amino acid (97-kDa) S-layer protein. Seque nce analysis of sapB indicated that the conserved N-terminal encoding region in sapA was absent but that the remainder of the ORF (encoding 751 amino acids) was identical to that of sapA in spite of the noncons erved nature of this region among sapA homologs. Noncoding sequences b oth 300 base pairs 5' and 1000 base pairs 3' to the sapB and sapA ORFs , including the sapA promoter and transcriptional terminator sequences , were essentially identical. Southern analyses revealed that the sapB N-terminal encoding region was conserved in multiple copies in type B strains but was absent in type A strains. Recombinant sapA and sapB p roducts bound to a substantially greater degree to cells of the homolo gous LPS type compared with the heterologous LPS type, indicating that the conserved sapA- and sapB-encoded N termini are critical for LPS b inding specificity. The parallel genetic organization and identity at the nucleotide level in both coding and noncoding regions for sap homo logs in types A and B cells indicates the necessity of both homolog co nservation and high fidelity DNA replication in the biology of sap div ersity.