CRISPR RNA isolated from the purified CSM complex from Sulfolobus solfataricus

The Clustered Regularly Interspaced Palindromic Repeats (CRISPR) system is an adaptive immune system in prokaryotes. Surveillance Complexes encoded by CRISPR-associated (cas) genes utilise small RNAs for homology-directed detection and subsequent degradation of invading genetic elements. CRISPR effector complexes have been classified into three main types (I-III). Type III effector complexes share the Cas10 subunit but are sub-classifed as type IIIA (CSM) and IIIB (CMR), depending on their specificity for DNA or RNA targets, respectively. The role of CSM in limiting the spread of conjugative plasmids in Staphylococcus epidermidis was first described in 2008. Here, we report a detailed investigation of the composition and structure of the CSM complex from the archaeon Sulfolobus solfataricus using a combination of Electron Microscopy, mass spectrometry and deep sequencing. This reveals a three-dimensional model for the CSM complex that includes a helical component strikingly reminiscent of the backbone structure of the Type I (Cascade) family.