Abstract
The type VI secretion system (T6SS) is a supramolecular complex involved in the delivery of potent toxins during bacterial competition. Pseudomonas aeruginosa possesses three T6SS gene clusters and several hcp and vgrG gene islands, the latter encoding the spike at the T6SS tip. The vgrG1b cluster encompasses seven genes whose organization and sequences are highly conserved in P. aeruginosa genomes, except for two genes that we called tse7 and tsi7 We show that Tse7 is a Tox-GHH2 domain nuclease which is distinct from other T6SS nucleases identified thus far. Expression of this toxin induces the SOS response, causes growth arrest and ultimately results in DNA degradation. The cytotoxic domain of Tse7 lies at its C terminus, while the N terminus is a predicted PAAR domain. We find that Tse7 sits on the tip of the VgrG1b spike and that specific residues at the PAAR-VgrG1b interface are essential for VgrG1b-dependent delivery of Tse7 into bacterial prey. We also show that the delivery of Tse7 is dependent on the H1-T6SS cluster, and injection of the nuclease into bacterial competitors is deployed for interbacterial competition. Tsi7, the cognate immunity protein, protects the producer from the deleterious effect of Tse7 through a direct protein-protein interaction so specific that toxin/immunity pairs are effective only if they originate from the same P. aeruginosa isolate. Overall, our study highlights the diversity of T6SS effectors, the exquisite fitting of toxins on the tip of the T6SS, and the specificity in Tsi7-dependent protection, suggesting a role in interstrain competition.