Nature doi:10.1038/nature23467 (2017)

Bacterial type II CRISPR–Cas systems are well suited for genome engineering because they target double-stranded DNA using a single effector nuclease, Cas9. In contrast, type III CRISPR–Cas systems have nuclease activities that target both DNA and RNA, mediated by a multiprotein interference complex. Type III CRISPR–Cas systems contain another ribonuclease, Csm6, that is not physically associated with the interference complex but can also degrade invader RNAs. Niewoehner et al. now show that the activities of the interference complex and Csm6 are linked by a cyclic oligoadenylate signaling molecule that is synthesized by Cas10 and allosterically activates Csm6. Structural data suggested that Csm6 might bind oligonucleotides, and the authors showed that synthetic oligoadenylates stimulated the RNase activity of Csm6 at a site in the enzyme's CARF domain. They further demonstrate that the natural regulatory nucleotide cyclic 3′,5′-linked hexaadenylate (cA6) is produced by the Cas10-containing interference complex in a reaction that requires ATP, Mg2+ and the Palm domain of Cas10. Studies in Csm6-knockout strains of Staphylococcus epidermidis provided evidence that the cyclase activity of Cas10 is required for regulating the RNA cleavage activity of Csm6 and bacterial immunity in vivo. By identifying this hidden activity of Cas10 and cA6 as a signaling molecule for type III systems, Niewoehner et al. highlight the diversity of bacterial CRISPR–Cas pathways and point to molecular regulators that may be adapted for biotechnological applications.