Diversity, classification and evolution of CRISPR-Cas systems

Curr Opin Microbiol. 2017 Jun;37:67-78. doi: 10.1016/j.mib.2017.05.008. Epub 2017 Jun 9.


The bacterial and archaeal CRISPR-Cas systems of adaptive immunity show remarkable diversity of protein composition, effector complex structure, genome locus architecture and mechanisms of adaptation, pre-CRISPR (cr)RNA processing and interference. The CRISPR-Cas systems belong to two classes, with multi-subunit effector complexes in Class 1 and single-protein effector modules in Class 2. Concerted genomic and experimental efforts on comprehensive characterization of Class 2 CRISPR-Cas systems led to the identification of two new types and several subtypes. The newly characterized type VI systems are the first among the CRISPR-Cas variants to exclusively target RNA. Unexpectedly, in some of the class 2 systems, the effector protein is additionally responsible for the pre-crRNA processing. Comparative analysis of the effector complexes indicates that Class 2 systems evolved from mobile genetic elements on multiple, independent occasions.

Publication types

  • Review

MeSH terms

  • Archaea / enzymology*
  • Archaea / genetics*
  • Bacteria / enzymology*
  • Bacteria / genetics*
  • CRISPR-Cas Systems*
  • Evolution, Molecular*
  • Genetic Variation*