Dual Expression Profile of Type VI Secretion System Immunity Genes Protects Pandemic Vibrio cholerae

Abstract

The Vibrio cholerae type VI secretion system (T6SS) assembles as a molecular syringe that injects toxic protein effectors into both eukaryotic and prokaryotic cells. We previously reported that the V. cholerae O37 serogroup strain V52 maintains a constitutively active T6SS to kill other Gram-negative bacteria while being immune to attack by kin bacteria. The pandemic O1 El Tor V. cholerae strain C6706 is T6SS-silent under laboratory conditions as it does not produce T6SS structural components and effectors, and fails to kill Escherichia coli prey. Yet, C6706 exhibits full resistance when approached by T6SS-active V52. These findings suggested that an active T6SS is not required for immunity against T6SS-mediated virulence. Here, we describe a dual expression profile of the T6SS immunity protein-encoding genes tsiV1, tsiV2, and tsiV3 that provides pandemic V. cholerae strains with T6SS immunity and allows T6SS-silent strains to maintain immunity against attacks by T6SS-active bacterial neighbors. The dual expression profile allows transcription of the three genes encoding immunity proteins independently of other T6SS proteins encoded within the same operon. One of these immunity proteins, TsiV2, protects against the T6SS effector VasX which is encoded immediately upstream of tsiV2. VasX is a secreted, lipid-binding protein that we previously characterized with respect to T6SS-mediated virulence towards the social amoeba Dictyostelium discoideum. Our data suggest the presence of an internal promoter in the open reading frame of vasX that drives expression of the downstream gene tsiV2. Furthermore, VasX is shown to act in conjunction with VasW, an accessory protein to VasX, to compromise the inner membrane of prokaryotic target cells. The dual regulatory profile of the T6SS immunity protein-encoding genes tsiV1, tsiV2, and tsiV3 permits V. cholerae to tightly control T6SS gene expression while maintaining immunity to T6SS activity.

A multidrug resistance plasmid contains the molecular switch for type VI secretion in Acinetobacter baumannii

“…This could indicate that A. baumannii is not capable of selftargeting or that the immunity proteins protecting from T6SS-mediated attacks are produced even under T6-conditions. Given findings in Vibrio cholerae, in which immunity proteins are transcribed constitutively and independently of other T6SS genes (27), we favor the latter hypothesis. This is further supported by our experiments showing that T6+ A. baumannii is able to kill nonkin T6-cells (Fig.…”

“…baumannii are resistant to T6SS-mediated attack from T6+ sister cells but not from nonkin T6+ A. baumannii. This indicates that the immunity proteins involved in preventing selfintoxication are produced even when the secretory apparatus itself is not expressed, analogous to the scenario described in V. cholerae (27). However, at this time the effector-immunity pairs of A. baumannii have not been characterized, and as such direct experimental evidence of this remains to be seen.…”

Bacteroides fragilis type VI secretion systems use novel effector and immunity proteins to antagonize human gut Bacteroidales species

“…Contraction of the sheath surrounding the needle apparatus drives expulsion of the needle from the cell, delivering the needle and associated effectors either into the supernatant of in vitro grown bacteria, or across the membrane of prey cells. Identified T6SS effectors include cell wall degrading enzymes (11), proteins that affect cell membranes such as phospholipases (12) and pore-forming toxins (13,14), proteins that degrade NAD(P)+ (15), and nucleases (16). The effector protein is produced with a cognate immunity protein, typically encoded by the adjacent downstream gene (17), which protects the producing cell from the toxicity of the effector.…”

The Self-Identity Protein IdsD Is Communicated between Cells in Swarming Proteus mirabilis Colonies

“…T6SSs, which are widely distributed among Gram-negative bacteria, are machines that can translocate proteins (primarily lethal) from the inside of one cell directly into another cell (8-28). The action of these transferred effector proteins is inhibited through the binding of an inhibitory immunity protein in the recipient cell (15,16,18,21,22,(28)(29)(30).In addition to a functional T6SS, the Ids system relies on the interactions between two proteins, D and IdsE (E), which together encode strain-specific identity information (2, 31). D and E each contain a variable region (VR), a stretch of amino acids that is generally unique among strains (2, 31).…”