Journal: Nature / Year: 2021 Title: Mechanism for Cas4-assisted directional spacer acquisition in CRISPR-Cas. Authors: Chunyi Hu / Cristóbal Almendros / Ki Hyun Nam / Ana Rita Costa / Jochem N A Vink / Anna C Haagsma / Saket R Bagde / Stan J J Brouns / Ailong Ke / Abstract: Prokaryotes adapt to challenges from mobile genetic elements by integrating spacers derived from foreign DNA in the CRISPR array. Spacer insertion is carried out by the Cas1-Cas2 integrase complex. A ...Prokaryotes adapt to challenges from mobile genetic elements by integrating spacers derived from foreign DNA in the CRISPR array. Spacer insertion is carried out by the Cas1-Cas2 integrase complex. 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) 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.
History
Deposition
Apr 16, 2021
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Header (metadata) release
Nov 17, 2021
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Map release
Nov 17, 2021
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Update
Nov 17, 2021
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Current status
Nov 17, 2021
Processing site: RCSB / Status: Released
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Structure visualization
Movie
Surface view with section colored by density value
Entire : Half integration complex and Cas4 dissociated from PAM side
Entire
Name: Half integration complex and Cas4 dissociated from PAM side
Components
Complex: Half integration complex and Cas4 dissociated from PAM side
Protein or peptide: half integration complex of Cas1-Cas2
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Supramolecule #1: Half integration complex and Cas4 dissociated from PAM side
Supramolecule
Name: Half integration complex and Cas4 dissociated from PAM side type: complex / ID: 1 / Parent: 0 / Macromolecule list: all / Details: half integration in cas4 containing system
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