Type II ribosome-independent endoribonuclease toxins and type III endoribonuclease toxins. Structures of type II ribosome-independent endoribonuclease toxins (A-E) and type III endoribonuclease toxins (F-H). Amino acid sequences for the toxin protein structures were from Escherichia coli strain K-12 unless otherwise indicated. (A) MazF monomer (PDB ID: 1UB4). Hydrophilic residues required for interacting with a substrate are indicated in olive green. Pro-30 is required for determining substrate specificity (green) and Trp-14 is also involved in substrate specificity (blue). (B) ChpBK monomer (PDB ID: 1M1F). (C) PemK monomer (PDB ID: 1M1F) of Staphylococcus aureus. (D) HicA monomer (PDB ID: 4P78). Two residues required for HicA activity are indicated in green. (E) MqsR monomer (PDB ID: 3HI2). MqsR toxin is the only member of RelE superfamily that has a ribosome-independent endoribonuclease activity. Four key residues are indicated in green. (F) ToxN monomer of Pectobacterium atrosepticum (PDB ID: 2XD0). Six residues required for ToxNPa’s activity are indicated in blue. (G) ToxN monomer of Bacillus thuringiensis (PDB ID: 4ATO). Six key residues are also indicated in pink. (H) AbiQ monomer of Lactococcus lactis (PDB ID: 4GLK). Seven key residues are indicated in pale violet.