This family includes Dmd peptides from T4 phages. Dmd can suppress the toxicities of toxins such as LsoA (an endoribonucleases toxin expressed by E.coli). Crystal structure analysis show that Dmd is inserted into the deep groove between the N-terminal repeated domain (NRD) and the Dmd-binding domain (DBD) of LsoA. Site-directed mutagenesis of Dmd revealed the conserved residues (W31 and N40) are necessary for LsoA binding and the toxicity suppression [].
Toxin/antitoxin (TA) systems are highly conserved among bacteria and archaea. They play a role in plasmid maintenance, biofilm formation, persister cell formation, general stress response, and phage defense and consist of a stable toxin and an unstable antitoxin. Toxins RnlA (RNase LS) and LsoA are type II TA systems that play a role in phage defence []. RnlA and LsoA have endoribonuclease activity, activated after bacteriophage T4 infection. RnlA is the stable toxin that causes inhibition of cell growth and rapidly degrades T4 late mRNAs to prevent their expression, and this is neutralised by the activity of the unstable antitoxin RnlB [, , ].The crystal structure of RnlA revealed that it consist of an N-terminal domain (NTD), an N-Repeated Domain (NRD) and a Dmd-Binding Domain (DBD), being this structure unique and different from all known toxin structures [].This entry represents the C-terminal DBD domain of RnlA and LsoA bacterial toxins, responsible for the RNA cleavage activity and toxicity. Through interaction between DBDs, RnlA dimerises in solution. T4 Dmd binds to DBD for inhibition of RnlA toxicity and RnlB also inhibits the toxicity of DBD [, ].
