A family of T6SS antibacterial effectors related to L,D-transpeptidases targets the peptidoglycan

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.