A tRNA-derived second messenger mediates antiviral defense
CRISPR-Cas systems are RNA-guided nucleases that enable prokaryotic immunity; however, some loci encode additional associated genes that cooperate with CRISPR effectors to perform diverse biological functions. Here, we uncover a CRISPR-associated kinase (CASK) system that links the recognition of target RNA to protein phosphorylation. We show that the kinase Csx33 phosphorylates Csx34 following activation of Cas13, enabling Csx34 to bind DNA in a sequence- specific manner. Together, Csx33 and Csx34 function as a transcriptional activation module that upregulates cas13 and associated genes, revealing a positive autoregulatory circuit that potentiates the immune response upon detection of foreign RNA. At the molecular level, Csx33 is activated by CCA trinucleotide RNA generated by Cas13-mediated cleavage of tRNA 3′ tails, uncovering a novel linear second messenger in bacterial immunity and a previously unrecognized signaling role of collateral RNA fragments. Together, these findings establish CASK systems as a new platform for RNA sensing and for engineering programmable phosphorylation-based signaling systems.