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.

VI型分泌系统（T6SS）是一种超分子复合物，参与细菌竞争过程中强效毒素的传递。铜绿假单胞菌具有三个T6SS基因簇和几个hcp和vgrG基因岛，后者编码T6SS尖端的尖峰。该vgrG1b集群包括7个基因，其组织和序列高度保守的铜绿假单胞菌基因组中，除了两个基因，我们称之为tse7和tsi7。我们显示，Tse7是Tox-GHH2域核酸酶，与迄今鉴定出的其他T6SS核酸酶不同。该毒素的表达诱导SOS反应，引起生长停滞并最终导致DNA降解。Tse7的细胞毒性结构域位于其C末端，而N末端是一个预测的PAAR结构域。我们发现Tse7位于VgrG1b尖峰的尖端，并且PAAR–VgrG1b界面上的特定残基对于Tse7依赖VgrG1b的向细菌猎物的传递至关重要。我们还显示，Tse7的传递取决于H1-T6SS簇，并且将核酸酶注射到细菌竞争剂中用于细菌间竞争。Tsi7，同源免疫蛋白，铜绿假单胞菌分离物。总体而言，我们的研究突出了T6SS效应子的多样性，毒素在T6SS尖端的精确匹配以及Tsi7依赖性保护的特异性，提示了在株间竞争中的作用。