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The CRISPR (clustered regularly interspaced short palindromic repeats) immune system in prokaryotes uses small guide RNAs to neutralize invading viruses and plasmids. In Escherichia coli, immunity depends on a ribonucleoprotein complex called Cascade. Here we present the composition and low-resolution structure of Cascade and show how it recognizes double-stranded DNA (dsDNA) targets in a sequence-specific manner. Cascade is a 405-kDa complex comprising five functionally essential CRISPR-associated (Cas) proteins (CasA(1)B(2)C(6)D(1)E(1)) and a 61-nucleotide CRISPR RNA (crRNA) with 5'-hydroxyl and 2',3'-cyclic phosphate termini. The crRNA guides Cascade to dsDNA target sequences by forming base pairs with the complementary DNA strand while displacing the noncomplementary strand to form an R-loop. Cascade recognizes target DNA without consuming ATP, which suggests that continuous invader DNA surveillance takes place without energy investment. The structure of Cascade shows an unusual seahorse shape that undergoes conformational changes when it binds target DNA.

Original publication

DOI

10.1038/nsmb.2019

Type

Journal article

Journal

Nat Struct Mol Biol

Publication Date

05/2011

Volume

18

Pages

529 - 536

Keywords

Base Sequence, Binding Sites, DNA, Escherichia coli, Escherichia coli Proteins, Nucleic Acid Conformation, Protein Structure, Tertiary, RNA, Bacterial, RNA, Guide, Ribonucleoproteins, Structure-Activity Relationship