Expression of the Helicobacter pylori virulence factor vacuolating cytotoxin A (vac A) is influenced by a potential stem‐loop structure in the 5′ untranslated region of the transcript

The vacuolating cytotoxin, VacA, is an important virulence factor secreted by the gastric pathogen H elicobacter pylori. Certain vac A genotypes are strongly associated with disease risk, but the association is not absolute. The factors determining vac A gene expression are not fully understood, and the mechanisms of its regulation are elusive. We have identified a potential mRNA stem‐loop forming structure in the 5′ untranslated region (UTR) of the vac A transcript. Using site‐directed mutagenesis, we found that disruption of the stem‐loop structure reduced steady‐state mRNA levels between two‐ and sixfold (P = 0.0005) and decreased mRNA half‐life compared with wild type (P = 0.03). This led to a marked reduction in VacA protein levels and overall toxin activity. Additionally, during stressful environmental conditions of acid pH or high environmental salt concentrations, when general transcription of vac A was decreased or increased respectively, the stabilising effects of the stem‐loop were even more pronounced. Our results suggest that the stem‐loop structure in the vac A 5′ UTR is an important determinant of vac A expression through stabilisation of the vac A mRNA transcript and that the stabilising effect is of particular importance during conditions of environmental stress.