Molecular identification and characterization of type III restriction-modification (R-M) gene cluster in Campylobacter lari

Abstract

Although the human clinical strain of Campylobacter lari (RM2100) has been shown not to carry any type III restriction-modification (R-M) systems, an R-M genes cluster was identified downstream of the full-length cytolethal distending toxin gene operon in the urease-positive thermophilic Campylobacter (UPTC) CF89-12 strain. Two possible open reading frames (ORFs) for restriction endonuclease and methyltransferase were predicted to encode peptides of 947 and 613 amino acid residues with calculated molecular weights of 111 and 70.8 kDa, respectively. Two putative promoters consisting of the consensus sequences and two probable ribosome binding sites for the two ORFs were also identified. Reverse transcription PCR identified co-transcription of the R-M genes in the cells. The existence of an S-adenosyl methionine-binding motif in the N-terminal conserved region of the possible ORF for the M gene, and seven conserved helicase motifs in the R gene were also identified. PCR and Southern blot hybridization analyses for type III R-M enzyme genes with some of the C. lari isolates including UPTC gave positive signals. UPTC isolates were shown to carry type III R-M enzyme genes, with a relatively high frequency.