Clustered, regularly interspaced, short palindromic repeat (CRISPR) loci play a pivotal role in the prokaryotic host defense system against invading genetic materials. The CRISPR loci are transcribed to produce CRISPR RNAs (crRNAs), which form interference complexes with CRISPR-associated (Cas) proteins to target the invading nucleic acid for degradation [1]. Four Cas proteins (Cas5, Cas6b, Cas7 and Cas8b) are proposed to form a Type I-B Cascade complex that mediates the antiviral defense [2]. This is the C-terminal domain found in Cas6b proteins. Cas6b is a member of Cas6 RNA processing endoribonucleases found in bacteria and archaea whose RNA substrates have a wide range of structural features. Cocrystal structures of Cas6 from Methanococcus maripaludis (MmCas6b) bound with its repeat RNA revealed a dual-site binding structure and a cleavage site conformation poised for phosphodiester bond breakage [3].

1: Crystal structure of the Csm3-Csm4 subcomplex in the type III-A CRISPR-Cas interference complex. Numata T, Inanaga H, Sato C, Osawa T; J Mol Biol. 2015;427:259-273. PMID:25451598

2: Fragmentation of the CRISPR-Cas Type I-B signature protein Cas8b. Richter H, Rompf J, Wiegel J, Rau K, Randau L; Biochim Biophys Acta. 2017; [Epub ahead of print] PMID:28238733

3: A Non-Stem-Loop CRISPR RNA Is Processed by Dual Binding Cas6. Shao Y, Richter H, Sun S, Sharma K, Urlaub H, Randau L, Li H; Structure. 2016;24:547-554. PMID:26996962