The REC lobe of Cas9 - the CRISPR-associated endonuclease Cas9 - includes the REC1 and REC2 domains. REC1 forms an elongated, alpha-helical structure consisting of 25 alpha helices and two beta-sheets, whereas REC2 inserted within REC1 adopts a six-helix bundle structure. The REC lobe and the NUC lobe of Cas9 fold to present a positively charged groove at their interface which accommodates the negatively charged sgRNA:target DNA heteroduplex [1]. CRISPR (clustered regularly interspaced short palindromic repeat)-Cas system occurs naturally in bacteria as a defence against invasion by phages or other mobile genetic elements. Cas9 is targeted to specific genomic locations by sgRNAs or single guide RNAs, in order to complex with invading DNA in order to cleave it and render it inactive.

1: Crystal Structure of Cas9 in Complex with Guide RNA and Target DNA. Nishimasu H, Ran FA, Hsu PD, Konermann S, Shehata SI, Dohmae N, Ishitani R, Zhang F, Nureki O; Cell. 2014; [Epub ahead of print] PMID:24529477