Ishita Jain, Matvey Kolesnik, Anna Shiriaeva, Sofia Medvedeva and Konstantin Severinov from Skoltech together with colleagues from Eugene Koonin’s lab published a preprint “tRNA anticodon cleavage by target-activated CRISPR-Cas13a effector”. Type VI CRISPR-Cas systems are the only CRISPR variety that cleaves exclusively RNA, Cas13, an effector protein in this type of CRISPR-Cas cystems, causes bacterial cell dormancy, thus protecting the host population from phage spread. The scientists showed that Cas13a protein from bacterium Leptotrichia shahii is capable of cleavage of anticodons of multiple tRNA species. This tRNA cleavage is necessary and sufficient for bacterial dormancy induction by Cas13a. Furthermore, Cas13a activates the RNases of bacterial toxin-antitoxin modules, thus indirectly causing mRNA and rRNA cleavage. Cleavage of bacterial RNAs induces dormant state. The described mode of action of Cas13a resembles that of bacterial anticodon nucleases involved in antiviral defense. Thus, one may suppose that that type VI effectors evolved from an abortive infection module encompassing an anticodon nuclease. Full text of the preprint is available here.