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

Abstract

Many bacteria kill rival species by translocating toxic effectors intotarget cells. Effectors are often encoded along with cognateimmunity proteins that could (i) protect against“friendly-fire”(trans-intoxication) from neighboring sister cells and/or (ii) pro-tect against internalcis-intoxication (suicide). Here, we distinguishbetween these two mechanisms in the case of the bactericidalXanthomonas citriType IV Secretion System (X-T4SS). We use aset ofX. citrimutants lacking multiple effector/immunity protein(X-Tfe/X-Tfi) pairs to show that X-Tfis are not absolutely requiredto protect againsttrans-intoxication by wild-type cells. Our inves-tigation then focused on the in vivo function of the lysozyme-likeeffector X-TfeXAC2609and its cognate immunity protein X-TfiXAC2610.In the absence of X-TfiXAC2610, we observe X-TfeXAC2609-dependentand X-T4SS-independent accumulation of damage in theX. citricellenvelope, cell death, and inhibition of biofilm formation. Whileimmunity proteins in other systems have been shown to protectagainst attacks by sister cells (trans-intoxication), this is anexample of an antibacterial secretion system in which the immunityproteins are dedicated to protecting cells againstcis-intoxication.