A tRNA-Acetylating Toxin and Detoxifying Enzyme in Mycobacterium tuberculosis

Toxin-antitoxin (TA) systems allow bacteria to adapt to changing environments without altering gene expression. Despite being overrepresented in Mycobacterium tuberculosis, their physiological roles remain elusive. We describe a TA system in M. tuberculosis which we named TacAT due to its homology to previously discovered systems in Salmonella. The toxin, TacT, blocks growth by acetylating glycyl-tRNAs and inhibiting translation. Its effects are reversed by the enzyme peptidyl tRNA hydrolase (Pth), which also cleaves peptidyl tRNAs that are prematurely released from stalled ribosomes. Pth is essential in most bacteria and thereby has been proposed as a promising drug target for complex pathogens like M. tuberculosis. Transposon sequencing data suggest that the tacAT operon is nonessential for M. tuberculosis growth in vitro, and premature stop mutations in this TA system present in some clinical isolates suggest that it is also dispensable in vivo. We assessed whether TacT modulates pth essentiality in M. tuberculosis because drugs targeting Pth might prompt resistance if TacAT is disrupted. We show that pth essentiality is unaffected by the absence of tacAT. These results highlight a fundamental aspect of mycobacterial biology and indicate that Pth’s essential role hinges on its peptidyl-tRNA hydrolase activity. Our work underscores Pth’s potential as a viable target for new antibiotics. IMPORTANCE The global rise in antibiotic-resistant tuberculosis has prompted an urgent search for new drugs. Toxin-antitoxin (TA) systems allow bacteria to adapt rapidly to environmental changes, and Mycobacterium tuberculosis encodes more TA systems than any known pathogen. We have characterized a new TA system in M. tuberculosis: the toxin, TacT, acetylates charged tRNA to block protein synthesis. TacT's effects are reversed by the essential bacterial enzyme peptidyl tRNA hydrolase (Pth), which is currently being explored as an antibiotic target. Pth also cleaves peptidyl tRNAs that are prematurely released from stalled ribosomes. We assessed whether TacT modulates pth essentiality in M. tuberculosis because drugs targeting Pth might prompt resistance if TacT is disrupted. We show that pth essentiality is unaffected by the absence of this TA system, indicating that Pth's essential role hinges on its peptidyl-tRNA hydrolase activity. Our work underscores Pth's potential as a viable target for new antibiotics.