Toxic component of a hybrid type II/IV toxin-antitoxin (TA) system. ADP-ribosylates ssDNA, preferentially in the motif TTTW. Its toxic effect is neutralized by cognate antitoxin DarG. a thymidine in DNA + NAD(+) = an N-(ADP-alpha-D-ribosyl)-thymidine in DNA + H(+) + nicotinamide Forms a complex with cognate antitoxin DarG; this complex neutralizes the toxic effect of DarT. By DNA damage (mitomycin C) as well as by darT overexpression. 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.