DNA Targeting by a Minimal CRISPR RNA-Guided Cascade
- Author(s): Hochstrasser, ML
- Taylor, DW
- Kornfeld, JE
- Nogales, E
- Doudna, JA
- et al.
Published Web Locationhttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC5111854/
© 2016 Elsevier Inc. Bacteria employ surveillance complexes guided by CRISPR (clustered, regularly interspaced, short palindromic repeats) RNAs (crRNAs) to target foreign nucleic acids for destruction. Although most type I and type III CRISPR systems require four or more distinct proteins to form multi-subunit surveillance complexes, the type I-C systems use just three proteins to achieve crRNA maturation and double-stranded DNA target recognition. We show that each protein plays multiple functional and structural roles: Cas5c cleaves pre-crRNAs and recruits Cas7 to position the RNA guide for DNA binding and unwinding by Cas8c. Cryoelectron microscopy reconstructions of free and DNA-bound forms of the Cascade/I-C surveillance complex reveal conformational changes that enable R-loop formation with distinct positioning of each DNA strand. This streamlined type I-C system explains how CRISPR pathways can evolve compact structures that retain full functionality as RNA-guided DNA capture platforms.
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