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

Summary AbiQ is a phage resistance mechanism found on a native plasmid of L actococcus 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. N orthern and W estern 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.