TRANSCRIPTIONAL ANALYSIS OF THE DIVERGENT CAGAB GENES ENCODED BY THE PATHOGENICITY ISLAND OF HELICOBACTER-PYLORI

Helicobacter pylori strains isolated from most patients with peptic ul cer disease and adenocarcinoma express the vacuolating toxin VacA and contain a pathogenicity island named cag. The cag pathogenicity island codes for more than 40 putative proteins with features similar to bac terial secretion systems. One of these proteins, CagA, is an immunodom inant antigen with unknown function encoded by the cagA gene. In the p resent study, we have analysed the functional promoter elements of the H. pylori cagA gene as well as of the divergently transcribed cagB ge ne. Primer extension analyses identified a single 5' end of the cagA m RNA, while two initiation sites were mapped in the case of the cagB mR NA. The promoters deduced upstream of these start points of transcript ion contained conserved -10 regions but no -35 regions with respect to the Escherichia coli sigma(70) consensus sequence. Nevertheless, they could be activated in E. coli and in vitro by purified E. coli RNA po lymerase. Deletion analyses indicated that the cagA and cagB genes are transcribed by overlapping promoters and that full activation require s sequences up to -70 and -96 respectively. Instead, basal transcripti on is likely to be mediated by -10 extended promoter-like sequences. R NA polymerase is able to bind the -40 to -60 region of the cagA promot er, and its binding is mediated by the alpha-subunit. This region rese mbles the UP elements of prokaryotic promoters in location, sequence a nd mechanism of interaction with the RNA polymerase. We discuss the fe atures of these promoters and propose that they could represent a clas s of minimum promoters, which ensures a basic level of transcription, while full activation requires regulatory elements or a defined promot er context.