Abstract
The bacterial type VI secretion system (T6SS) is a supra-molecular complex akin to bacteriophage tails, with VgrG proteins acting as a puncturing device. The Pseudomonas aeruginosa H1-T6SS has been extensively characterized. It is involved in bacterial killing and in the delivery of three toxins, Tse1-3. Here, we demonstrate the independent contribution of the three H1-T6SS co-regulated vgrG genes, vgrG1abc, to bacterial killing. A putative toxin is encoded in the vicinity of each vgrG gene, supporting the concept of specific VgrG/toxin couples. In this respect, VgrG1c is involved in the delivery of an Rhs protein, RhsP1. The RhsP1 C terminus carries a toxic activity, from which the producing bacterium is protected by a cognate immunity. Similarly, VgrG1a-dependent toxicity is associated with the PA0093 gene encoding a two-domain protein with a putative toxin domain (Toxin_61) at the C terminus. Finally, VgrG1b-dependent killing is detectable upon complementation of a triple vgrG1abc mutant. The VgrG1b-dependent killing is mediated by PA0099, which presents the characteristics of the superfamily nuclease 2 toxin members. Overall, these data develop the concept that VgrGs are indispensable components for the specific delivery of effectors. Several additional vgrG genes are encoded on the P. aeruginosa genome and are not linked genetically to other T6SS genes. A closer inspection of these clusters reveals that they also encode putative toxins. Overall, these associations further support the notion of an original form of secretion system, in which VgrG acts as the carrier.
Highlights
The type VI secretion system (T6SS) is involved in bacterial warfare
A Broad Repertoire of Toxins Involved in H1-T6SS-dependent Killing—P. aeruginosa H1-T6SS secretes a set of three toxins, namely Tse1–3 [24], and the producing strains are protected from the activity of the Tse by cognate immunity proteins, Tsi1–3
The evolved V. cholerae VgrG1, displaying a C-terminal extension bearing an actin cross-linking domain (ACD), has been considered for a long time as the archetype substrate injected by the T6SS into eukaryotic cells (50 –53)
Summary
The type VI secretion system (T6SS) is involved in bacterial warfare. Results: Novel H1-T6SS-dependent toxins from Pseudomonas aeruginosa are identified using an in vitro bacterial killing assay. The VgrG1b-dependent killing is mediated by PA0099, which presents the characteristics of the superfamily nuclease 2 toxin members Overall, these data develop the concept that VgrGs are indispensable components for the specific delivery of effectors. Three gene couples have been shown to be controlled by the RetS signaling pathways and encode toxin/immunity pairs involved in H1-T6SS-dependent bacterial killing [23,24,25]. In the present study we lend support to this hypothesis by showing that the three VgrG proteins co-expressed with the H1-T6SS individually contribute to the toxicity exerted by a retS strain against Escherichia coli targets [34] This toxicity is observable in a background devoid of the characterized Tse toxins, revealing a broader repertoire of T6SS toxins
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