7mi5
Asymmetrical PAM-Non PAM prespacer bound Cas4/Cas1/Cas2 complex
Structural highlights
FunctionCS4F1_GEOSL 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) (By similarity). The Cas4 region acts as a ssDNA exonuclease, while the Cas1 region acts as a dsDNA endonuclease. Involved in the integration of spacer DNA into the CRISPR cassette (By similarity).[UniProtKB:Q97TX9] Publication Abstract from PubMedProkaryotes adapt to challenges from mobile genetic elements by integrating spacers derived from foreign DNA in the CRISPR array(1). Spacer insertion is carried out by the Cas1-Cas2 integrase complex(2-4). A substantial fraction of CRISPR-Cas systems use a Fe-S cluster containing Cas4 nuclease to ensure that spacers are acquired from DNA flanked by a protospacer adjacent motif (PAM)(5,6) and inserted into the CRISPR array unidirectionally, so that the transcribed CRISPR RNA can guide target searching in a PAM-dependent manner. Here we provide a high-resolution mechanistic explanation for the Cas4-assisted PAM selection, spacer biogenesis and directional integration by type I-G CRISPR in Geobacter sulfurreducens, in which Cas4 is naturally fused with Cas1, forming Cas4/Cas1. During biogenesis, only DNA duplexes possessing a PAM-embedded 3'-overhang trigger Cas4/Cas1-Cas2 assembly. During this process, the PAM overhang is specifically recognized and sequestered, but is not cleaved by Cas4. This 'molecular constipation' prevents the PAM-side prespacer from participating in integration. Lacking such sequestration, the non-PAM overhang is trimmed by host nucleases and integrated to the leader-side CRISPR repeat. Half-integration subsequently triggers PAM cleavage and Cas4 dissociation, allowing spacer-side integration. Overall, the intricate molecular interaction between Cas4 and Cas1-Cas2 selects PAM-containing prespacers for integration and couples the timing of PAM processing with the stepwise integration to establish directionality. Mechanism for Cas4-assisted directional spacer acquisition in CRISPR-Cas.,Hu C, Almendros C, Nam KH, Costa AR, Vink JNA, Haagsma AC, Bagde SR, Brouns SJJ, Ke A Nature. 2021 Oct;598(7881):515-520. doi: 10.1038/s41586-021-03951-z. Epub 2021 , Sep 29. PMID:34588691[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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