Toxic component of a type II toxin-antitoxin (TA) system. Acts as an endoribonuclease on single-strand RNA, cleaving between the first and second bases in the sequence UCGCU. Neutralized by coexpression with cognate antitoxin MazE1. Forms a complex with cognate antitoxin MazE1. Belongs to the PemK/MazF family.