Sequence- and Structure-Specific RNA Processing by a CRISPR Endonuclease

CRISPR Processing Many bacteria and archaea recognize invading viruses and plasmids. Foreign DNA is integrated into so-called clustered regularly interspaced short palindromic repeat (CRISPR) loci, and transcripts from these loci are processed into RNAs that can target the invading DNA or RNA for destruction. To investigate the molecular basis for this processing, Haurwitz et al. (p. 1355 ) screened CRISPR-associated (Cas) proteins in the opportunistic pathogen Pseudomonas aeruginosa and found they were capable of cleaving the CRISPR transcripts. The crystal structure of Cas4 with the CRISPR RNA transcript revealed how the protein specifically recognized RNA repeats, as well as the mechanism of endonucleolytic cleavage.