Toxic component of a type II toxin-antitoxin (TA) system. Phosphorylates UDP-N-acetyl-D-glucosamine (UNAG) on the 3'-hydroxyl group of the N-acetyl-D-glucosamine moiety, yielding UNAG-3P. UNAG-3P inhibits MurA, the first committed step in cell wall synthesis, which is then blocked. Phosphorylation is inhibited by cognate epsilon antitoxin. Part of a postsegregational killing (PSK) system involved in the killing of plasmid-free cells. The zeta toxin induces programmed cell death. ATP + UDP-N-acetyl-alpha-D-glucosamine = ADP + H(+) + UDP-N-acetyl-alpha-D-glucosamine 3'-phosphate In the presence of the epsilon antitoxin forms an inactive PezA(2)PezT(2) heterotetramer. The heterotetramer is still able to bind the UNAG substrate. Has a long half-life in vivo. Belongs to the zeta toxin family.