CRISPR–Cas (clustered, regularly interspaced short palindromic repeats–CRISPR-associated proteins) loci of bacteria and archaea encode small CRISPR RNAs (crRNAs) and Cas proteins that assemble into effector complexes to target and destroy invading nucleic acid in a sequence-specific manner. CRISPR–Cas systems are classified into three types (I, II and III), and several subtypes, according to the set of Cas proteins that they contain; type II systems function with a single Cas9 protein, whereas types I and III contain several different Cas proteins. Previous work has mainly focused on the characterization of type I systems, which form a complex known as Cascade. Now, three studies report the structural characterization of type III CRISPR–Cas complexes from Sulfolobus solfataricus, Pyrococcus furiosus and Thermus thermophilus. Electron microscopy shows that, despite having divergent Cas proteins and crRNAs, the complexes from all three organisms have a similar overall shape, which consists of a crRNA-binding helical backbone that resembles the seahorse shape of Cascade. This striking architectural similarity between type I and type III systems reveals that distantly related CRISPR–Cas complexes share a common functional design, which suggests that they evolved from a common ancestor. Furthermore, functional insights into the nucleolytic activity of type III systems were also uncovered. Rouillon et al. show that the type III-A complex from S. solfataricus can bind but cannot cleave target double-stranded DNA, which suggests that a nuclease must be recruited to the complex. This is also reminiscent of Cascade, which functions as a surveillance complex and recruits a distinct nuclease for target degradation. By contrast, Staals et al. find that the type III-B complex of T. thermophilus has nucleolytic activity, although the exact component of the complex responsible for target degradation is unclear. Collectively, these studies provide unparalleled insight into the composition and function of type III CRISPR–Cas complexes, which can now be exploited for further biochemical characterization of CRISPR-mediated interference.
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