Toxic component of a hybrid type II/IV toxin-antitoxin (TA) system. ADP-ribosylates ssDNA on the second thymidine of the consensus sequence 5'-TNTC-3'; the protein does not auto-modify. Arg-51 is highly flexible, allowing it to assume multiple positions in the crystal structures (PubMed:34408320). Its toxic effect is neutralized by cognate antitoxin DarG (By similarity). a thymidine in DNA + NAD(+) = an N-(ADP-alpha-D-ribosyl)-thymidine in DNA + H(+) + nicotinamide Probably forms a complex with cognate antitoxin DarG; this complex might decrease ADP-ribosylation activity of DarT. The NAD(+)-binding element stabilizes the ADP-ribosylating turn-turn (ARTT) loop which confers substrate specificity; both domains contribute to ssDNA-binding. Belongs to the DarT ADP-ribosyltransferase family.