Molecular Characterization and Interstrain Variability of pHPS1, a Plasmid Isolated from the Sydney Strain (SS1) ofHelicobacter pylori

Abstract

The 5846-bp circular plasmid pHPS1 ofHelicobacter pyloriSydney strain, SS1, was cloned, sequenced, and structurally characterized. The SS1 strain is widely used in animal studies ofH. pyloriinfection. The sequence of pHPS1 revealed three open reading frames (ORFs), all of which are transcribed. Two ORFs encode putative plasmid replication proteins, RepA and RepB, similar to replicases resident on theta plasmids. In contrast, the function of ORF2 remains cryptic due to the absence of sequence similarity with any known protein in sequence databases. In addition, species specificity of these three coding regions was shown using DNA dot blot hybridization in 57 diverse clinicalH. pyloriisolates and 32HelicobacterandCampylobacterstrains. RepA appears to be the predominant plasmid replication protein ofH. pyloriand the deduced amino acid sequence was highly conserved (76–96%) in 8H. pyloriisolates, including SS1. RepB was detected in 3H. pyloriisolates examined in this study, 2 of which possess only therepBgene. Analysis of the protein sequences of these two replicases, together with previously characterizedH. pyloriplasmid replication proteins, supports the formation of a distinct class ofH. pyloriplasmid proteins. Moreover, comprehensive analysis of the whole genome sequence ofH. pyloristrain 26695, pHPS1, and otherH. pyloriplasmid sequences that are available revealed interesting insights as to the occurrence of plasmid-mediated recombination withinH. pylori.Common regions between plasmids and chromosome sequences ofH. pyloriwere identified in this study which could only have arisen by genetic recombination, thus providing the first line of evidence, albeit indirectly, of the contribution ofH. pyloriplasmids in generating an extensive genetic heterogeneity characteristic of this important gastroduodenal pathogen.