Journal Article

Unravelling the structural and mechanistic basis of CRISPR–Cas systems

Westra, Edze R., Wiedenheft, Blake, Jackson, Ryan N., van der Oost, John

2014

Overview

Key Points CRISPR–Cas (clustered regularly interspaced short palindromic repeats–CRISPR-associated proteins) is an adaptive immune system in bacteria and archaea, which is categorized into three distinct types (known as type I, type II and type III) that differ in their compositions and mechanisms of action. Adaptive immunity occurs in three distinct stages: acquisition, expression and interference. In all three types of CRISPR–Cas system, spacer acquisition relies on a pair of conserved Cas proteins (Cas1 and Cas2), which are probably assisted by a variable set of additional (potentially both Cas and non-Cas) enzymes. Primary processing of CRISPR RNAs (crRNAs) is catalysed either by Cas6-like ribonucleases (in the type I and type III systems) or by RNase III (which targets the crRNA and transactivating crRNA (tracrRNA) duplex in type II systems). Mature crRNAs form CRISPR ribonucleoprotein (crRNP) complexes by associating with either Cascade-like multiprotein complexes (in type I and type III systems) or the multidomain Cas9 protein (in type II systems). Discrimination of 'self' nucleic acid from 'non-self' nucleic acid enables crRNP complexes to specifically target invading nucleic acid (usually DNA). Some complexes recruit additional nucleases (for example, Cas3 in type I systems), whereas other crRNPs have intrinsic nuclease domains (for example, Cas9 in type II systems). Fundamental studies have elucidated many mechanistic features of CRISPR–Cas functionality by integrating genetics, biochemistry and structural biology. This has provided an excellent basis for developing a wide range of applications, from the manipulation of gene expression in bacteria to genome editing in eukaryotes. In this Review, van der Oost et al . summarize the recent structural and biochemical insights into the molecular mechanisms of RNA-guided interference by CRISPR–Cas systems in bacteria and archaea. By comparing the three main types of CRISPR–Cas systems, they highlight the unique and conserved properties of the system and also discuss outstanding questions that require further study. Bacteria and archaea have evolved sophisticated adaptive immune systems, known as CRISPR–Cas (clustered regularly interspaced short palindromic repeats–CRISPR-associated proteins) systems, which target and inactivate invading viruses and plasmids. Immunity is acquired by integrating short fragments of foreign DNA into CRISPR loci, and following transcription and processing of these loci, the CRISPR RNAs (crRNAs) guide the Cas proteins to complementary invading nucleic acid, which results in target interference. In this Review, we summarize the recent structural and biochemical insights that have been gained for the three major types of CRISPR–Cas systems, which together provide a detailed molecular understanding of the unique and conserved mechanisms of RNA-guided adaptive immunity in bacteria and archaea.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/326/26/2527 / 631/326/41/1969 / 631/326/88 / 631/45/535 / Acids / Adaptive immunity / Archaea - genetics / Archaea - immunology / Bacteria / Bacteria - genetics / Bacteria - immunology / bacterial immune-system / Clustered Regularly Interspaced Short Palindromic Repeats - physiology / CRISPR / CRISPR-Cas Systems - genetics / CRISPR-Cas Systems - physiology / crystal-structure / Deoxyribonucleic acid / DNA / Enzymes / escherichia-coli / Genetic aspects / Genetic research / human gut virome / Identification and classification / Immune system / Infectious Diseases / interference complex / Life Sciences / Medical Microbiology / Microbial genetics / Microbiological research / Microbiology / Models, Molecular / Nucleic acids / Parasitology / Plasmids - genetics / processes pre-crrna / Proteins / Quantitative trait loci / review-article / Ribonucleoproteins - chemistry / Ribonucleoproteins - physiology / rna silencing complex / RNA, Bacterial - physiology / short palindromic repeats / streptococcus-thermophilus / Surveillance / thermus-thermophilus / Virology / Viruses / Viruses - genetics