Specificity and mechanism of tRNA cleavage by the AriB Toprim nuclease of the PARIS bacterial immune system.

Transfer RNA (tRNA) molecules have been recently recognized as widespread targets of bacterial immune systems. Translation inhibition through tRNA cleavage or modification inhibits phage propagation, thereby protecting the bacterial population. To counteract this, some viruses encode their own tRNA molecules, allowing infection to take place. The AriB effector of the PARIS defence system is a Toprim nuclease previously shown to target the Escherichia coli tRNA(Lys(UUU)), but not a tRNA(Lys(UUU)) variant encoded by bacteriophage T5. We demonstrate here that the T5 tRNA(Lys(UUU)) is required but not sufficient to bypass PARIS immunity. Combining tRNA sequencing, genetics, phage infection and in vitro biochemical data, we reveal that the E. coli tRNA(Thr(UGU)) is another prime target of AriB, and tRNA(Asn(GUU)) represents a secondary, yet biologically relevant, target of the PARIS effector. Activated AriB protein cleaves these targets in vitro, and the cleavage reaction is not dependent on the presence of specific tRNA modifications. We show that the overexpression of phage T5 tRNA(Lys(UUU)), tRNA(Thr(UGU)) and tRNA(Asn(GUU)) variants is sufficient to inhibit PARIS anti-viral defence. Finally, we propose a model for tRNA recognition by the AriB dimer and provide molecular details of its nuclease activity and specificity.This article is part of the discussion meeting issue 'The ecology and evolution of bacterial immune systems'.