RNAinDefense:CRISPRsProtectProkaryotes against Mobile Genetic Elements

SUMMARY The CRISPR/Cas system in prokaryotes provides resistance against invading viruses and plasmids. Three distinct stages in the mechanismcan berecognized. Initially, fragments of invader DNAareintegratedasnewspacersintotherepetitiveCRISPRlocus.Subsequently,theCRISPR istranscribed and the transcript is cleaved bya Cas protein within the repeats, generating short RNAs (crRNAs) that contain the spacer sequence. Finally, crRNAs guide the Cas protein machinerytoacomplementary invader target,either DNAorRNA,resultingininhibitionofvirus orplasmidproliferation.Inthisarticle,wediscussourcurrentunderstandingofthisfascinating adaptive and heritable defense system, and describe functional similarities and differences with RNAi in eukaryotes.