A nucleotidyltransferase toxin inhibits growth of <i>Mycobacterium tuberculosis</i> through inactivation of tRNA acceptor stems.

Abstract

Toxin-antitoxin systems are widespread stress-responsive elements, many of whose functions remain largely unknown. Here, we characterize the four DUF1814-family nucleotidyltransferase-like toxins (MenT_1-4) encoded by the human pathogen <i>Mycobacterium tuberculosis</i>. Toxin MenT₃ inhibited growth of <i>M. tuberculosis</i> when not antagonized by its cognate antitoxin, MenA₃. We solved the structures of toxins MenT₃ and MenT₄ to 1.6 and 1.2 Å resolution, respectively, and identified the biochemical activity and target of MenT₃. MenT₃ blocked in vitro protein expression and prevented tRNA charging in vivo. MenT₃ added pyrimidines (C or U) to the 3'-CCA acceptor stems of uncharged tRNAs and exhibited strong substrate specificity in vitro, preferentially targeting tRNA^Ser from among the 45 <i>M</i>. <i>tuberculosis</i> tRNAs. Our study identifies a previously unknown mechanism that expands the range of enzymatic activities used by bacterial toxins, uncovering a new way to block protein synthesis and potentially treat tuberculosis and other infections.