Toxic component of a type II toxin-antitoxin (TA) system. Specific for 5'-UACAU-3' sequences, cleaving after the first U (PubMed:21763692). Yields cleavage products with 3' phosphate and 5' hydroxyl groups (PubMed:15882409). Cannot digest substrate with a UUdUACAUAA cleavage site (PubMed:24120662). Overexpression is toxic for cell growth (shown in E.coli), probably by inhibiting protein synthesis through the cleavage of single-stranded RNA. The toxicity is reversed by the antitoxin EndoAI. Toxin activity cannot be inhibited by MazE from E.coli. The EndoA-EndoAI complex does not seem to bind its own promoter (PubMed:24120662). Homodimer (PubMed:14517982, PubMed:24120662). Forms a complex with antitoxin EndoAI in which the toxin activity is inhibited (PubMed:15882409). One dimer binds a ssRNA substrate, forms a heterohexamer composed of alternating toxin and antitoxin homodimers which inhibits the endoribonuclease activity. Antitoxin prevents RNA binding to the endoribonuclease (PubMed:24120662). Replacing all Arg residues by canavanine yields an enzyme that recognizes a longer consensus sequence UACAUA, thus altering its substrate specificity. Still cleaves between the first and second nucleotides. Belongs to the PemK/MazF family.