Research Article

Crystal structure of a CRISPR RNA–guided surveillance complex bound to a ssDNA target

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Science  19 Sep 2014:
Vol. 345, Issue 6203, pp. 1479-1484
DOI: 10.1126/science.1256996

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A foreign-DNA–destroying machine

Bacteria have an adaptive immune system, called CRISPR, that identifies invading viruses through their DNA or RNA sequences and cuts them up (see the Perspective by Zhang and Sontheimer). Jackson et al. and Mulepati et al. have determined the structure of the large protein complex, called Cascade, that targets the invading nucleic acids and does the cutting. The seahorse-shaped structure reveals how the 11 subcomponents of Cascade assemble into the final protein complex. The structure also shows how Cascade presents the short CRISPR-derived RNAs so that they can bind and target foreign DNA.

Science, this issue p. 1473 and p. 1479; see also p. 1452

Abstract

In prokaryotes, RNA derived from type I and type III CRISPR loci direct large ribonucleoprotein complexes to destroy invading bacteriophage and plasmids. In Escherichia coli, this 405-kilodalton complex is called Cascade. We report the crystal structure of Cascade bound to a single-stranded DNA (ssDNA) target at a resolution of 3.03 angstroms. The structure reveals that the CRISPR RNA and target strands do not form a double helix but instead adopt an underwound ribbon-like structure. This noncanonical structure is facilitated by rotation of every sixth nucleotide out of the RNA-DNA hybrid and is stabilized by the highly interlocked organization of protein subunits. These studies provide insight into both the assembly and the activity of this complex and suggest a mechanism to enforce fidelity of target binding.

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