Toxic component of a type II toxin-antitoxin (TA) system. Acts as an endoribonuclease (mRNA interferase) on single-strand mRNA, cleaving between the first and second bases in the sequence UCGCU. Overexpression in M.smegmatis but not E.coli inhibits growth, this effect is neutralized by coexpression with cognate toxin MazE4. Residues 29-56 inhibit ssDNA cleavage by DNA topoisomerase. This fragment does not have mRNA cleavage activity but it inhibits growth upon overexpression in M.smegmatis. RNA cleavage is inhibited by the C-terminal domain of DNA topoisomerase I. Forms a complex with cognate antitoxin MazE4 (By similarity). Interacts with DNA topoisomerase I (PubMed:20724443). Belongs to the PemK/MazF family.