CRISPR-associated endonuclease Cas9

UniProtKB accession:  G3ECR1

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UniProtKB description:  CRISPR (clustered regularly interspaced short palindromic repeat) is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). In type II CRISPR systems correct processing of pre-crRNA requires a trans-encoded small RNA (tracrRNA), endogenous ribonuclease 3 (rnc) and Cas9. The tracrRNA serves as a guide for ribonuclease 3-aided processing of pre-crRNA (Probable). Cas9/crRNA/tracrRNA endonucleolytically cleaves linear or circular dsDNA target complementary to the spacer yielding blunt ends; Cas9 is inactive in the absence of the 2 guide RNAs (gRNA). Cas9 recognizes a 3'-G-rich protospacer adjacent motif (PAM, TGGTG in this organism) in the CRISPR repeat sequences to help distinguish self versus nonself, as targets within the bacterial CRISPR locus do not have PAMs. PAM recognition is also required for catalytic activity. When the CRISPR3/cas system consisting of cas9-cas1-cas2-csn2-CRISPR3 or just cas9-CRISPR3 is expressed in E.coli it prevents plasmids homologous to spacers 1 or 2 from transforming.