The CRISPR‐Cas system: small RNA‐guided invader silencing in prokaryotes

Abstract

CRISPR‐Cas systems are recently discovered, RNA‐based immune systems that control invasions of viruses and plasmids in archaea and bacteria. Prokaryotes with CRISPR‐Cas immune systems capture short invader sequences within the CRISPR loci in their genomes, and produce small RNAs from the CRISPR loci (crRNAs) that recognize the invading nucleic acids and guide Cas proteins to degrade (or otherwise silence) the invader. There are multiple variations of the pathway mediated by distinct components and mechanisms that we are now delineating.Pyrococcus furiosus has 3 CRISPR‐Cas systems populated by 27 Cas proteins and 200 crRNAs. We have found that CRISPR transcripts are “diced” into individual invader‐targeting crRNAs by Cas6, a sequence‐specific riboendonuclease. The mature crRNAs contain a signature sequence element that is important for function. The CRISPR‐Cas systems in P. furiosus target invaders at both the RNA and DNA level. We have identified 3 distinct potential crRNA‐Cas protein effector complexes (arising from the 3 systems), and have determined that one, the Cmr complex, cleaves complementary RNAs at fixed intervals along the region of complementarity between the crRNA and target RNA. We have also initiated efforts to direct Cmr complexes to cleave novel target RNAs (a la prokaryotic RNAi).This work was supported by NIH grant RO1GM54682 (including ARRA funds) to B.T. and M.T.