Toxic component of a type II toxin-antitoxin (TA) system. Ribosome-independent, sequence-specific endoribonuclease that cleaves mRNA, thus inhibiting protein synthesis and inducing bacterial stasis. It cuts between the first and nucleotides of 5'-UACAU-3' in single-stranded RNA. Neutralized by coexpression with cognate antitoxin MazE. Homodimer (Ref.2). Forms a complex with MazE which is no longer active as an endoribonuclease. Belongs to the PemK/MazF family.