Llontop, E. E. | Search bioRxiv

bioRxiv2021

Antibacterial T6SS effectors with a VRR-Nuc domain induce target cell death via DNA Double-Strand Breaks

The T6SS (Type VI secretion System) secretes antibacterial effectors into target competitors. Salmonella spp. encode five phylogenetically distinct T6SSs. Here we characterize the function of the SPI-22 T6SS of S. bongori, showing that it has antibacterial activity. We identify a group of antibacterial T6SS effectors (TseV1-4) containing an N-terminal PAAR-like domain and a C-terminal VRR-Nuc domain encoded next to cognate immunity proteins that contain the DUF3396 domain (TsiV1-4). TseV2 and TseV3 are toxic when expressed in Escherichia coli and bacterial competition assays confirm that TseV2 and TseV3 are secreted by the SPI-22 T6SS. Phylogenetic analysis reveals that TseV1-4 are evolutionarily related to enzymes involved in DNA repair. TseV2 and TseV3 maintained the ability to bind DNA, but instead cause specific DNA double-strand breaks and induce the SOS response in target cells. The crystal structure of the TseV3:TsiV3 complex reveals that the immunity protein likely blocks the effector interaction with the DNA substrate. These results expand our knowledge on the function of Salmonella pathogenicity islands, the evolution of toxins used in biological conflicts, and the endogenous mechanism regulating the activity of these toxins.

microbiology↗

bioRxiv2021

Resource sharing by outer membrane vesicles from a citrus pathogen

The causative agent of citrus canker disease, Xanthomonas citri pv. citri, was found to produce copious amounts of outer membrane vesicles (OMVs), frequently forming long membranous tubes under different culture conditions. Lipidomic analysis revealed significant differences in lipid composition between purified vesicles in relation to whole cells. The results suggest an enrichment in saturated cardiolipins and a decrease in unsaturated lipids in the OMV samples, possibly granting them a more rigid structure while allowing their high degree of curvature caused by their small diameters. The vesicles proteome was found to be significantly enriched in TonB-dependent receptors related to the acquisition of different nutrients. These proteins are known to transport siderophores, which were evidenced to be present in purified X. citri OMVs, along with essential metals including iron, zinc, and manganese quantified by elemental analysis. The availability of vesicle-associated nutrients to be incorporated by cells was demonstrated by the use of OMVs as the sole carbon source for bacterial growth. At last, the vesicles also presented esterase and protease activities, which have been associated with virulence in phytopathogens. These evidences point that X. citri cells can use OMVs to share resources within microbial communities, which has potential implications for microbial interactions and plant colonization, affecting their survival and persistence on the host and in the environment. ImportanceThe shedding of outer membrane vesicles appears to be universal in Gram-negative bacteria and effectively constitutes a unique secretion pathway for diverse molecules and proteins. To study their possible functions in the citrus pathogen Xanthomonas citri, purified vesicles from this bacterium were studied by omics and functional approaches. Nutrient transporters were found associated to these structures, which were evidenced to contain siderophores and essential metals. The availability of these nutrients to be incorporated by cells was then demonstrated by showing that purified vesicles can be used as sole carbon sources for microbial growth. Additionally, the samples also presented esterase and protease activities which can contribute to the release of substrates from plant host tissues. These observations help to establish the developing idea of vesicles as shared bacterial resources which can participate in shaping host-associated microbial communities in contrast to other interactions such as bacterial competition.

microbiology↗