Abstract
Bacteria deliver toxic effectors via type VI secretion systems (T6SSs) to dominate competitors, but the identity and function of many effectors remain unknown. Here we identify a Vibrio antibacterial T6SS effector that contains a previously undescribed, widespread DNase toxin domain that we call PoNe (Polymorphic Nuclease effector). PoNe belongs to a diverse superfamily of PD-(D/E)xK phosphodiesterases, and is associated with several toxin delivery systems including type V, type VI, and type VII. PoNe toxicity is antagonized by cognate immunity proteins (PoNi) containing DUF1911 and DUF1910 domains. In addition to PoNe, the effector contains a domain of unknown function (FIX domain) that is also found N-terminal to known toxin domains and is genetically and functionally linked to T6SS. FIX sequences can be used to identify T6SS effector candidates with potentially novel toxin domains. Our findings underline the modular nature of bacterial effectors harboring delivery or marker domains, specific to a secretion system, fused to interchangeable toxins.
Highlights
Bacteria deliver toxic effectors via type VI secretion systems (T6SSs) to dominate competitors, but the identity and function of many effectors remain unknown
Effectors found outside of the main T6SS gene clusters in vibrios often belong to auxiliary T6SS modules that encode at least one of the tail tube components: VgrG, Hcp, or PAAR33,34
We found the VgrG1b module in the genomes of other V. parahaemolyticus strains isolated around the world (Fig. 1b and Supplementary Table 1)
Summary
Bacteria deliver toxic effectors via type VI secretion systems (T6SSs) to dominate competitors, but the identity and function of many effectors remain unknown. We identify a Vibrio antibacterial T6SS effector that contains a previously undescribed, widespread DNase toxin domain that we call PoNe (Polymorphic Nuclease effector). The T6SS is a multi-protein machine that is often encoded by large gene clusters[3] It delivers toxins, called effectors, across the bacterial cell envelope and into neighboring cells[4,5]. The baseplate serves as a platform for assembling the tail tube This secreted tube is composed of stacked hexameric rings of a protein called Hcp; these rings are capped by a spike composed of a VgrG trimer and a PAAR repeat-containing protein that sharpens the tip[1,2,8,9,10]. Many toxins that have been identified as T6SS effectors do not contain a recognizable delivery domain or signal, suggesting that additional delivery domains remain to be discovered
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