Abstract
Type VI secretion systems (T6SSs) are nanomachines used by bacteria to inject toxic effectors into competitors. The identity and mechanism of many effectors remain unknown. We characterized a Salmonella T6SS antibacterial effector called Tlde1 that is toxic in target-cell periplasm and is neutralized by its cognate immunity protein (Tldi1). Microscopy analysis reveals that cells expressing Tlde1 stop dividing and lose cell envelope integrity. Bioinformatic analysis uncovers similarities between Tlde1 and the catalytic domain of l,d-transpeptidases. Point mutations on conserved catalytic residues abrogate toxicity. Biochemical assays reveal that Tlde1 displays both l,d-carboxypeptidase activity by cleaving peptidoglycan tetrapeptides between meso-diaminopimelic acid3 and d-alanine4 and l,d-transpeptidase exchange activity by replacing d-alanine4 by a non-canonical d-amino acid. Phylogenetic analysis shows that Tlde1 homologs constitute a family of T6SS-associated effectors broadly distributed among Proteobacteria. This work expands our current knowledge about bacterial effectors used in interbacterial competition and reveals a different 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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