Type IV-C CRISPR-Cas effector complexes recognize double-stranded DNA and switch on collateral cleavage of ssDNA and RNA.
Type IV-C CRISPR-Cas systems remain enigmatic compared to other class 1 systems. Here, we expand the type IV-C catalog, identifying two phylogenetically distinct clades primarily found in archaea (IV-C1) or bacteria (IV-C2), distinguishable by the Cas10IVc subunit architecture. We functionally and structurally characterize type IV-C1 systems from Thermococcus onnurineus (Ton) and Pyrococcus abyssi (Pab). Type IV-C complexes assemble with crRNAs derived from distinct CRISPR arrays and recognize a 5'-GGG-3' protospacer adjacent motif (PAM) to bind double-stranded DNA targets. Target recognition activates the HD domain of Cas10IVc, triggering metal-dependent collateral cleavage of single-stranded DNA and RNA. This behavior is explained by allosteric alignment of the HD active site, triggered by PAM-dependent R-loop formation, as revealed by cryo-EM. Together, our findings suggest that type IV-C systems provide immunity via non-specific cleavage of nucleic acids generated during mobile genetic element replication or transcription.