Study of the functionality of the Helicobacter pylori trans-translation components SmpB and SsrA in an heterologous system

BACKGROUND: Trans-translation is a ubiquitous bacterial quality control-mechanism for both transcription and translation. With its two major partners, SsrA a small stable RNA and the SmpB protein, it promotes the release of ribosomes stalled on defective mRNAs and directs the corresponding truncated proteins to degradation pathways. We have recently shown that trans-translation is an essential function in the gastric pathogen Helicobacter pylori. Our results suggested that some properties of the H. pylori trans-translation machinery distinguishes it from the well known system in E. coli. Therefore, we decided to test the functionality of the SmpB and SsrA molecules of H. pylori in the E. coli heterologous system using two established phenotypic tests. RESULTS: H. pylori SmpB protein was found to successfully restore the E. coli ΔsmpB mutant growth defect and its capacity to propagate λimm(P22 )phage. We showed that in E. coli, H. pylori SsrA (Hp-SsrA) was stably expressed and maturated and that this molecule could restore wild type growth to the E. coli ΔssrA mutant. Hp-SsrA mutants affected in the ribosome rescue function were not able to restore normal growth to E. coli ΔssrA supporting a major role of ribosome rescue in this phenotype. Surprisingly, Hp-SsrA did not restore the phage λimm(P22 )propagation capacity to the E. coli ΔssrA mutant. CONCLUSIONS: These data suggest an additional role of the tag sequence that presents specific features in Hp-SsrA. Our interpretation is that a secondary role of protein tagging in phage propagation is revealed by heterologous complementation because ribosome rescue is less efficient. In conclusion, tmRNAs present in all eubacteria, have coevolved with the translational machinery of their host and possess specific determinants that can be revealed by heterologous complementation studies.