Toxic component of a hybrid type II/IV toxin-antitoxin (TA) system. Its toxic effect is neutralized by cognate antitoxin DarG. ADP-ribosylates ssDNA on the second thymidine of the consensus sequence 5'-TNTC-3'; the protein does not auto-modify. Has no activity on dsDNA in vitro. This leads to a decrease in DNA replication. Upon expression in E.coli inhibits cell growth, colony formation and induces the SOS response. Expression leads to bacteriostasis; however if cells grow over an hour in the presence of toxin, growth is no longer restored on antitoxin-inducing plates (PubMed:27939941). In E.coli ADP-ribosylates genomic DNA (gDNA), which induces RecA expression (a marker for DNA damage) (PubMed:34408320). 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. Truncated N-terminus.