AtaT blocks translation initiation by N-acetylation of the initiator tRNAfMet

Toxin-antitoxin (TA) loci are prevalent in bacterial genomes. They are suggested to play a central role in dormancy and persister states. Under normal growth conditions, TA toxins are neutralized by their cognate antitoxins, and under stress conditions, toxins are freed and inhibit essential cellular processes using a variety of mechanisms. Here we characterize ataR-ataT, a novel TA system, from enterohemorrhagic Escherichia coli. We show that the toxin AtaT is a GNAT family enzyme that transfers an acetyl group from acetyl coenzyme A to the amine group of the methionyl aminoacyl moiety of initiator tRNA. AtaT specifically modifies Met-tRNA(fMet), but no other aminoacyl-tRNAs, including the elongator Met-tRNAMet. We demonstrate that once acetylated, AcMet-tRNA(fMet) fails to interact with initiation factor-2 (IF2), resulting in disruption of the translation initiation complex. This work reveals a new mechanism of translation inhibition and confirms Met-tRNA(fMet) as a prime target to efficiently block cell growth.