Abstract
Type IV secretion systems (T4SSs) are multiprotein complexes that transport effector proteins and protein-DNA complexes through bacterial membranes to the extracellular milieu or directly into the cytoplasm of other cells. Many bacteria of the family Xanthomonadaceae, which occupy diverse environmental niches, carry a T4SS with unknown function but with several characteristics that distinguishes it from other T4SSs. Here we show that the Xanthomonas citri T4SS provides these cells the capacity to kill other Gram-negative bacterial species in a contact-dependent manner. The secretion of one type IV bacterial effector protein is shown to require a conserved C-terminal domain and its bacteriolytic activity is neutralized by a cognate immunity protein whose 3D structure is similar to peptidoglycan hydrolase inhibitors. This is the first demonstration of the involvement of a T4SS in bacterial killing and points to this special class of T4SS as a mediator of both antagonistic and cooperative interbacterial interactions.
Highlights
Type IV secretion systems (T4SSs) are multiprotein complexes that transport effector proteins and protein–DNA complexes through bacterial membranes to the extracellular milieu or directly into the cytoplasm of other cells
We show that these Xanthomonas VirD4-interacting proteins (XVIPs) are toxins secreted by the Xanthomonas T4SS, that secretion is dependent on the XVIPCD and that this special T4SS is used by Xanthomonas to kill other bacterial cells, thereby providing a competitive growth advantage in mixed bacterial communities
Six Xanthomonas citri (Xac) XVIPs (XAC0466, XAC0574, XAC1918, XAC2609, XAC2885 and XAC3634) are predicted to act within the periplasm as peptidoglycan (PG)-binding proteins, PG glycohydrolases, lytic transglycosylases, PG peptidases or lipases (Fig. 1a). The genes of these proteins, plus one other containing an HExxH metallopeptidase motif (XAC0096), are found downstream to, and potentially co-transcribed with, genes that code for proteins with unknown functions with predicted lipoprotein signal peptides that are expected to localize them within the bacterial periplasm (Fig. 1)
Summary
Type IV secretion systems (T4SSs) are multiprotein complexes that transport effector proteins and protein–DNA complexes through bacterial membranes to the extracellular milieu or directly into the cytoplasm of other cells. Many bacteria of the family Xanthomonadaceae, which occupy diverse environmental niches, carry a T4SS with unknown function but with several characteristics that distinguishes it from other T4SSs. Here we show that the Xanthomonas citri T4SS provides these cells the capacity to kill other Gram-negative bacterial species in a contact-dependent manner. These interactions have driven the evolution of several mechanisms by which they quickly deploy proteinaceous and nucleic acid effectors that manipulate the behaviour of the target organism, often resulting in growth inhibition or death1–6 Distinct among these mechanisms are the type III, type IV and type VI secretion systems (T3SS, T4SS and T6SS, respectively) that are all capable of transferring proteins, and in the case of the T4SS, protein–DNA complexes, directly into neighbouring cells in a contact-dependent manner. We show that these XVIPs are toxins secreted by the Xanthomonas T4SS, that secretion is dependent on the XVIPCD and that this special T4SS is used by Xanthomonas to kill other bacterial cells, thereby providing a competitive growth advantage in mixed bacterial communities
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