J. Biol. Chem. 288, 27888–27897 (2013)

The CRISPR-Cas systems make up an adaptive immune response used by bacteria and archaea to fend off invaders such as phage and foreign plasmid DNA. CRISPRs are clustered regularly interspaced short palindromic repeats harboring sequences acquired from past invaders that encode small antisense RNAs (crRNAs). To form the crRNAs, long precursor RNAs are first cleaved to intermediate lengths by a Cas endonuclease and undergo maturation, leaving a variable 3′ end that matches the foreign genome target that directs antisense-targeting mechanism. To understand this final step better in Staphylococcus epidermidis, Hatoum-Aslan et al. identified, by affinity chromatography, a five-protein complex containing known components of the S. epidermidis CRISPR-Cas system, which they named Cas10–Csm, as being involved in crRNA maturation. The purified complex contained a 71-nucleotide (nt) intermediate crRNA and a series of six mature crRNAs differing by 6 nt and could generate mature crRNAs from intermediate ones. Using a mutational analysis, the authors found that the Csm3 component of the complex was responsible for the observed crRNA size distribution in the ribonucleoprotein complex. In vitro, each Csm3 protein bound 6 nt of substrate and protected it from the cleavage involved in maturation. These results suggest that Csm3 acts as a ruler that measures the extent of crRNA maturation within the Cas10–Csm complex.