Extension of tRNA acceptor stems by MenT nucleotidyltransferase toxins of Mycobacterium tuberculosis and inhibition by asymmetrical toxin-antitoxin complex formation

Mycobacterium tuberculosis, the bacterium responsible for human tuberculosis, encodes a remarkably high number of toxin-antitoxin systems of largely unknown function. We have recently shown that M. tuberculosis encodes four of a new, widespread, MenAT family of nucleotidyltransferase toxin-antitoxin systems. In this study we characterize MenAT1, using tRNA sequencing to demonstrate MenT1 tRNA modification activity. MenT1 activity is blocked by MenA1, a short protein antitoxin unrelated to the MenA3 kinase. X-ray crystallographic analysis shows blockage of the conserved MenT fold by asymmetric binding of MenA1 across two MenT1 protomers, forming a heterotrimeric toxin-antitoxin complex. Finally, we also demonstrate tRNA modification by toxin MenT4, indicating conserved activity across the MenT family. Our study highlights variation in tRNA target preferences by MenT toxins, selective use of nucleotide substrates, and diverse modes of MenA antitoxin activity.