Toxic component of a toxin-antitoxin (TA) module. 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.