Mechanistic insights into type I toxin antitoxin systems inHelicobacter pylori:the importance of mRNA folding in controlling toxin expression

Abstract

Type I toxin-antitoxin (TA) systems have been identified in a wide range of bacterial genomes. Here, we report the characterization of a new type I TA system present on the chromosome of the major human gastric pathogen, Helicobacter pylori. We show that the aapA1 gene encodes a 30 amino acid peptide whose artificial expression in H. pylori induces cell death. The synthesis of this toxin is prevented by the transcription of an antitoxin RNA, named IsoA1, expressed on the opposite strand of the toxin gene. We further reveal additional layers of post-transcriptional regulation that control toxin expression: (i) transcription of the aapA1 gene generates a full-length transcript whose folding impedes translation (ii) a 3΄ end processing of this message generates a shorter transcript that, after a structural rearrangement, becomes translatable (iii) but this rearrangement also leads to the formation of two stem-loop structures allowing formation of an extended duplex with IsoA1 via kissing-loop interactions. This interaction ensures both the translation inhibition of the AapA1 active message and its rapid degradation by RNase III, thus preventing toxin synthesis under normal growth conditions. Finally, a search for homologous mRNA structures identifies similar TA systems in a large number of Helicobacter and Campylobacter genomes.

Mechanisms of Bacterial Tolerance and Persistence in the Gastrointestinal and Respiratory Environments

“…A new type I TA system, the AapA1/IsoA1 locus, present on the chromosome of H. pylori, was recently characterized (374). Several type II TA systems associated with the bacterial persistence phenotype have also been localized in this pathogen, including (i) the HP0894-HP0895 proteins (375,376), (ii) the HP0892-HP0893 proteins (RelE-family TA system), and (iii) the HP0967-HP0968 proteins (Vap family) (374).…”

“…These systems were encoded by the pVir plasmid, which is involved in virulence and natural transformation. VirT belongs to the RelE family, one of the best-studied TA systems (374,377).…”

Pathogenesis of Helicobacter pylori infection

“…A global RNA transcript mapping study revealed a number of putative type I toxin‐antitoxin (TA) systems . New work demonstrated rapid killing by the small peptide toxin of these systems, possibly by depleting ribosomal RNAs, and established a fascinating mechanism of post‐translational regulation . The aapA1 toxin full‐length transcript is not competent for translation due to long‐range base pairing interactions between the 3′ and 5′ ends.…”

“…42 New work demonstrated rapid killing by the small peptide toxin of these systems, possibly by depleting ribosomal RNAs, and established a fascinating mechanism of post-translational regulation. 43 The aapA1 toxin full-length transcript is not competent for translation due to long-range base pairing interactions between the 3′ and 5′ ends. This transcript is processed by an unknown nuclease leading to an altered secondary structure that is translation competent.…”

A peptide of a type I toxin-antitoxin system induces Helicobacter pylori morphological transformation from spiral-shape to coccoids

“…Four families (A-B-C-D) of conserved type I TA systems are expressed from the chromosome of H. pylori, only the A family was studied as it is highly expressed and conserved among H. pylori strains (23)(24)(25). For the A1 TA system, the detailed mechanism by which transcription of the IsoA1 antitoxin RNA impairs AapA1 toxin synthesis by base pairing with its primary transcript, ensuring both translation inhibition of the AapA1 active message and leading to rapid degradation of the duplex by RNase III, has been recently published (24,25). The H. pylori type I toxins are typically small hydrophobic peptides of 30-40 amino acids predicted to form alpha-helices (26).…”