CRISPR/Cas, the Immune System of Bacteria and Archaea

CRISPR Defenses Prokaryotes can be infected by parasites and pathogens and, like eukaryotes, have evolved systems to protect themselves. Horvath and Barrangou (p. 167 ) review a recently discovered prokaryotic “immune system” characterized by CRISPR—clustered regularly interspaced short palindromic repeats—found in most archaeal and many bacterial species. CRISPR loci harbor short sequences captured from viruses and invasive genetic elements. These sequences are transcribed, and the RNA is cleaved into short CRISPR RNAs (crRNAs) by one of a family of CRISPR-associated (cas) proteins. These crRNAs direct other cas family proteins to homologous nucleic acid targets to effect their destruction. Through its ability to impede the spread of specific nucleic acid sequences, the CRISPR/Cas systems might be exploited to block the dissemination of antibiotic-resistance markers.