Abstract
Type VI secretion systems (T6SSs) are contractile nanomachines widely used by bacteria to intoxicate competitors. Salmonella Typhimurium encodes a T6SS within the Salmonella pathogenicity island 6 (SPI-6) that is used during competition against species of the gut microbiota. We characterized a new SPI-6 T6SS antibacterial effector named Tlde1 (type VI L,D-transpeptidase effector 1). Tlde1 is toxic in target-cell periplasm and its toxicity is neutralized by co-expression with immunity protein Tldi1 (type VI L,D-transpeptidase immunity 1). Time-lapse microscopy revealed that intoxicated cells display altered cell division and lose cell envelope integrity. Bioinformatics analysis showed that Tlde1 is evolutionarily related to L,D-transpeptidases. Point mutations on conserved histidine121 and cysteine131 residues eliminated toxicity. Co-incubation of purified recombinant Tlde1 and peptidoglycan tetrapeptides showed that Tlde1 displays both L,D-carboxypeptidase activity by cleaving GM-tetrapeptides between meso-diaminopimelic acid3 and D-alanine4, and L,D-transpeptidase exchange activity by replacing D-alanine4 for a non-canonical D-amino acid. Tlde1 constitutes a new family of T6SS effectors widespread in Proteobacteria. This work increases our knowledge about the bacterial effectors used in interbacterial competitions and provides molecular insight into a new mechanism of bacterial antagonism.
Highlights
Bacteria commonly live in densely populated polymicrobial communities and compete over scarce resources
Our study reveals a different mechanism for effector-mediated bacterial antagonism and indicates that Tlde1 targets the peptidoglycan synthesis in two ways: (1) the L,D-CPase activity reduces the amount of acceptor tetrapeptide stems, reducing the formation of new crosslinks by D,D-TPases; and (2) the L,D-TPase exchange activity promotes the incorporation of non-canonical D-amino acid (NCDAA) into tetrapeptides affecting their recycling and reducing the availability of cell wall precursors and substrates for D,D-transpeptidation
Tlde1-Tldi1 Are an Antibacterial Effector-Immunity Pair To search for new T6SS effectors secreted by S
Summary
Bacteria commonly live in densely populated polymicrobial communities and compete over scarce resources. The type VI secretion system (T6SS) is a dynamic contractile structure evolutionarily related to bacteriophage tails that delivers protein effectors in a contact-dependent manner into diverse cellular types, including eukaryotic host cells and rival bacteria and fungi (Hachani et al, 2016; Coulthurst, 2019; Trunk et al, 2019). A conformational change in the T6SS baseplate is thought to trigger the contraction of a cytoplasmic sheath, expelling a spear-like structure to puncture target cell membranes (Wang et al, 2017; Salih et al, 2018). Along with the Hcp-VgrG-PAAR puncturing device, a cocktail of effectors is delivered into the target cell after each contraction event
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