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Mol Cell. 2013 Oct 10;52(1):135-145. doi: 10.1016/j.molcel.2013.09.013.

Structure and activity of the RNA-targeting Type III-B CRISPR-Cas complex of Thermus thermophilus.

Author information

Laboratory of Microbiology, Department of Agrotechnology and Food Sciences, Wageningen University, 6703 HB Wageningen, The Netherlands.
RIKEN SPring-8 Center, 679-5148 Hyogo, Japan.
Department of Molecular Biophysics and Biochemistry, Yale University School of Medicine, New Haven, CT 06520-8114, USA.
California Institute for Quantitative Biosciences, University of California, San Francisco, CA 94158-2330, USA.
Biomolecular Mass Spectrometry and Proteomics, Bijvoet Center for Biomolecular Research and Utrecht Institute for Pharmaceutical Sciences, University of Utrecht, 3584 CH Utrecht, The Netherlands.
Laboratory of Systems and Synthetic Biology, Wageningen University, 6703 HB Wageningen, The Netherlands.
Global Research Cluster, RIKEN, Saitama 351-0198, Japan.
Department of Molecular and Cell Biology, and Howard Hughes Medical Institute, University of California, Berkeley, CA 94720-3200, USA; Physical Biosciences Division, Lawrence Berkeley National Lab, Berkeley, CA 94720-3200, USA.
Contributed equally


The CRISPR-Cas system is a prokaryotic host defense system against genetic elements. The Type III-B CRISPR-Cas system of the bacterium Thermus thermophilus, the TtCmr complex, is composed of six different protein subunits (Cmr1-6) and one crRNA with a stoichiometry of Cmr112131445361:crRNA1. The TtCmr complex copurifies with crRNA species of 40 and 46 nt, originating from a distinct subset of CRISPR loci and spacers. The TtCmr complex cleaves the target RNA at multiple sites with 6 nt intervals via a 5' ruler mechanism. Electron microscopy revealed that the structure of TtCmr resembles a "sea worm" and is composed of a Cmr2-3 heterodimer "tail," a helical backbone of Cmr4 subunits capped by Cmr5 subunits, and a curled "head" containing Cmr1 and Cmr6. Despite having a backbone of only four Cmr4 subunits and being both longer and narrower, the overall architecture of TtCmr resembles that of Type I Cascade complexes.

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