Xanthomonas immunity proteins protect against the cis-toxic effects of their cognate T4SS effectors

Many bacteria kill rival species by translocating toxic effectors into target cells. Effectors are often encoded along with cognate immunity proteins that could (i) protect against “friendly-fire” ( trans -intoxication) from neighboring sister cells and/or (ii) protect against internal cis -intoxication (suicide). Here, we distinguish between these two mechanisms in the case of the bactericidal Xanthomonas citri Type IV Secretion System (X-T4SS). We use a set of X. citri mutants lacking multiple effector/immunity protein (X-Tfe/X-Tfi) pairs to show that X-Tfis are not absolutely required to protect against trans -intoxication by wild-type cells. Our investigation then focused on the in vivo function of the lysozyme-like effector X-Tfe XAC2609 and its cognate immunity protein X-Tfi XAC2610 . In the absence of X-Tfi XAC2610 , we observe X-Tfe XAC2609 -dependent and X-T4SS-independent accumulation of damage in the X. citri cell envelope, cell death, and inhibition of biofilm formation. While immunity proteins in other systems have been shown to protect against attacks by sister cells ( trans -intoxication), this is an example of an antibacterial secretion system in which the immunity proteins are dedicated to protecting cells against cis -intoxication. Synopsis Immunity proteins of bactericidal Xanthomonadaceae Type IV Secretion Systems (X-T4SSs) are dedicated to protecting cells against cis -intoxication (suicide) instead of trans -intoxication (fratricide) by their cognate effectors. Bactericidal X-T4SSs translocate toxic effectors to kill bacteria of other species in a contact-dependent manner. Immunity proteins are not required to protect against the toxic effects of effectors translocated by neighbouring sister cells. Immunity proteins do protect against the toxic effects of endogenously produced effectors. The absence of one specific immunity protein leads to the accumulation of damage in the bacterial cell envelope. Immunity proteins of bactericidal Xanthomonadaceae Type IV Secretion Systems (X-T4SSs) are dedicated to protecting cells against cis -intoxication (suicide) instead of trans -intoxication (fratricide) by their cognate effectors.