Toxic component of a type II toxin-antitoxin (TA) system. A sequence-specific endoribonuclease, it cuts between the first and second nucleotides of 5'-UACAU-3' (PubMed:19251861, PubMed:23994560). Cleaves the artificial ssDNA-RNA substrate 5'-AAGTCrUrACATCAG-3' between rU and rA in vitro (PubMed:23994560). Originally found to be a ribosome-independent, with a consensus sequence of 5'-[ACG]UU[ACG]-3' in single-stranded RNA (PubMed:17933891, PubMed:19168622). Its overexpression inhibits protein synthesis and induces bacterial stasis, it is neutralized by coexpression with cognate antitoxin MazE (PubMed:17933891, PubMed:19168622, PubMed:19251861). Not all mRNAs are equally susceptible in vivo (gyrB, recA and sarA for example are not degraded), while no degradation of rRNA has been seen (PubMed:19168622). Forms a complex with MazE which is no longer active as an endoribonuclease. By heat shock and by exposure to sub-MIC concentrations of several antimicrobial agents such as doxycycline, erythromycin and penicillin. The cleavage efficiency was significantly reduced if the first or fourth residues of the consensus region were changed to U. Alterations in any of the two U residues in the center of the consensus sequence completely abolished the cleavage by MazF. Belongs to the PemK/MazF family.