Depletion of the DarG antitoxin in Mycobacterium tuberculosis triggers the DNA-damage response and leads to cell death.

Of the ~80 putative toxin-antitoxin (TA) modules encoded by the bacterial pathogen Mycobacterium tuberculosis (Mtb), three contain antitoxins essential for bacterial viability. One of these, Rv0060 (DNA ADP-ribosyl glycohydrolase, DarG ), functions along with its cognate toxin Rv0059 (DNA ADP-ribosyl transferase, DarT ), to mediate reversible DNA ADP-ribosylation (Jankevicius et al., 2016). We demonstrate that DarT -DarG form a functional TA pair and essentiality of darG is dependent on the presence of darT , but simultaneous deletion of both darT -darG does not alter viability of Mtb in vitro or in mice. The antitoxin, DarG , forms a cytosolic complex with DNA-repair proteins that assembles independently of either DarT or interaction with DNA. Depletion of DarG alone is bactericidal, a phenotype that is rescued by expression of an orthologous antitoxin, DarG , from Thermus aquaticus. Partial depletion of DarG triggers a DNA-damage response and sensitizes Mtb to drugs targeting DNA metabolism and respiration. Induction of the DNA-damage response is essential for Mtb to survive partial DarG -depletion and leads to a hypermutable phenotype.