Structure and activity of AbiQ,a lactococcal endoribonuclease belonging to the type III toxin–antitoxin system

AbiQ is a phage resistance mechanism found on a native plasmid of Lactococcus lactis that abort virulent phage infections. In this study, we experimentally demonstrate that AbiQ belongs to the recently described type III toxin–antitoxin systems. When overexpressed, the AbiQ protein (ABIQ) is toxic and causes bacterial death in a bacteriostatic manner. Northern and Western blot experiments revealed that the abiQ gene is transcribed and translated constitutively, and its expression is not activated by a phage product. ABIQ is an endoribonuclease that specifically cleaves its cognate antitoxin RNA molecule in vivo. The crystal structure of ABIQ was solved and site‐directed mutagenesis identified key amino acids for its anti‐phage and/or its RNase function. The AbiQ system is the first lactococcal abortive infection system characterized to date at a structural level.