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A Cas9–guide RNA complex preorganized for target DNA recognition

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Science  26 Jun 2015:
Vol. 348, Issue 6242, pp. 1477-1481
DOI: 10.1126/science.aab1452

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An RNA seed poised to meet its target

The CRISPR-Cas system in prokaryotes precisely identifies infecting parasitic DNAs and viruses and destroys them. The CRISPR-Cas system has been adapted for facile genome editing, heralding a new age in molecular biology. Jiang et al. show that the Cas9 nuclease adopts a distinct confirmation when it binds to the targeting guide RNA. The guide RNA then assumes a preordered shape. This RNA “seed region” is thus poised to initiate recognition of the DNA target sequence.

Science, this issue p. 1477

Abstract

Bacterial adaptive immunity uses CRISPR (clustered regularly interspaced short palindromic repeats)–associated (Cas) proteins together with CRISPR transcripts for foreign DNA degradation. In type II CRISPR-Cas systems, activation of Cas9 endonuclease for DNA recognition upon guide RNA binding occurs by an unknown mechanism. Crystal structures of Cas9 bound to single-guide RNA reveal a conformation distinct from both the apo and DNA-bound states, in which the 10-nucleotide RNA “seed” sequence required for initial DNA interrogation is preordered in an A-form conformation. This segment of the guide RNA is essential for Cas9 to form a DNA recognition–competent structure that is poised to engage double-stranded DNA target sequences. We construe this as convergent evolution of a “seed” mechanism reminiscent of that used by Argonaute proteins during RNA interference in eukaryotes.

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