Zhu, X.; Morency, C.; Picard, M.-E.; Mosterd, C.; McAlister, J. A.; Perrault-Jolicoeur, A.; Shi, R.; Moineau, S.

Prokaryotes and eukaryotes possess defense systems, which can be either innate or acquired, to protect against viral infections. At the bacterial population level, abortive infection (Abi) serves as an innate immune defense mechanism against phage invasion. The AbiV antiviral system is prevalent in several bacterial genomes and exhibits diverse characteristics in terms of gene composition and evolution. Our investigation into the Lactococcus AbiV system revealed a novel two-component system, abiV1 and abiV2, both of which are essential for its function as a type III toxin-antitoxin system. The toxin component AbiV (product of abiV1) is an RNase belonging to the HEPN (Higher Eukaryotes and Prokaryotes Nucleotide-binding) superfamily as it carries the consensus Rx4-6H motif. In vivo assays coupled with mass spectrometry showed that the lactococcal AbiV was expressed in the presence or absence of phages while in vitro experiments demonstrated that AbiV1 degraded ribosomal RNA but not mRNA. On the other hand, the antitoxin component (abiV2) was found to function as an RNA molecule that inhibited the nuclease activity of the AbiV1 toxin. The structural characterization of AbiV revealed that this RNase utilizes a large patch of positively charged area across the dimer to anchor RNA molecules. In addition, we showed that the AbiV N-terminal region (amino acids 1 to 23) is crucial for its RNase activity as a truncated AbiV lacking this segment adopted distinct conformational states incompatible with RNA binding. This study provided novel insights into the mode of action of the antiviral system AbiV.