Molecular characterization and interstrain variability of pHPS1, a plasmidisolated from the Sydney strain (SS1) of Helicobacter pylori

The 5846-bp circular plasmid pHPS1 of Helicobacter pylori Sydney strain, SS 1, was cloned, sequenced, and structurally characterized. The SS1 strain is widely used in animal studies of H. pylori infection. The sequence of pHPS 1 revealed three open reading frames (ORFs), all of which are transcribed. Two ORFs encode putative plasmid replication proteins, RepA and RepB, simil ar to replicases resident on theta plasmids. In contrast, the function of O RF2 remains cryptic due to the absence of sequence similarity with any know n protein in sequence databases. In addition, species specificity of these three coding regions was shown using DNA dot blot hybridization in 57 diver se clinical H. pylori isolates and 32 Helicobacter and Campylobacter strain s. RepA appears to be the predominant plasmid replication protein of H. pyl ori and the deduced amino acid sequence was highly conserved (76-96%) in 8 H. pylori isolates, including SS1. RepB was detected in 3 H. pylori isolate s examined in this study, 2 of which possess only the repB gene. Analysis o f the protein sequences of these two replicases, together with previously c haracterized H. pylori plasmid replication proteins, supports the formation of a distinct class of H. pylori plasmid proteins. Moreover, comprehensive analysis of the whole genome sequence of H. pylori strain 26695, pHPS1, an d other H. pylori plasmid sequences that are available revealed interesting insights as to the occurrence of plasmid-mediated recombination within H. pylori. Common regions between plasmids and chromosome sequences of H. pylo ri were identified in this study which could only have arisen by genetic re combination, thus providing the first line of evidence, albeit indirectly, of the contribution of H. pylori plasmids in generating an extensive geneti c heterogeneity characteristic of this important gastroduodenal pathogen. ( C) 1999 Academic Press.