Characterization of a 3.5-kbp plasmid from Helicobacter pylori

A 3.5-kbp plasmid (pHPM180) was isolated from Helicobacter pylori (HPM180) and the DNA sequence was determined. Two open reading frames (ORF1 and ORF2) were identified which could encode polypeptides of 54,517 and 27,629 Da, respectively. Ribosome binding and promoter consensus sequences were identified, as well as two 232-bp direct repeats and four 22-bp direct repeats. DNA sequence homology was found between pHPM180 ORF1 and a 684-base pair HindIII fragment from a 5.6-kbp H. pylori plasmid. ORF1 showed amino acid homology with six replication proteins from bacterial plasmids with theta-type replicons. Additional sequence identity was found between pHPM180 noncoding DNA and a segment of H. pylori pHPK255, a plasmid that replicates via a rolling circle type mechanism. A ribonuclease protection assay determined that ORF1 was transcribed in H. pylori HPM180.