Toxic component of a type II toxin-antitoxin (TA) system, cannot be expressed in E.coli in the absence of cognate antitoxin MazE (PubMed:24748664). 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 (By similarity). Its endoribnuclease activity is neutralized by coexpression with cognate antitoxin MazE (PubMed:24748664). Thermostable at 90 degrees Celsius. Homodimer. Forms a complex with MazE which inhibits the endoribonuclease activity of MazF. Belongs to the PemK/MazF family.