Mechanism for Cas4-assisted directional spacer acquisition in CRISPR–Cas

Chunyi Hu, Cristóbal Almendros, Ki Hyun Nam, Ana Rita Costa, Jochem N. A. Vink, Anna C. Haagsma, Saket R. Bagde, Stan J. J. Brouns () and Ailong Ke ()
Additional contact information
Chunyi Hu: Cornell University
Cristóbal Almendros: Delft University of Technology
Ki Hyun Nam: Pohang University of Science and Technology
Ana Rita Costa: Delft University of Technology
Jochem N. A. Vink: Delft University of Technology
Anna C. Haagsma: Delft University of Technology
Saket R. Bagde: Cornell University
Stan J. J. Brouns: Delft University of Technology
Ailong Ke: Cornell University

Nature, 2021, vol. 598, issue 7881, 515-520

Abstract: Abstract Prokaryotes adapt to challenges from mobile genetic elements by integrating spacers derived from foreign DNA in the CRISPR array1. Spacer insertion is carried out by the Cas1–Cas2 integrase complex2–4. 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.

Date: 2021
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DOI: 10.1038/s41586-021-03951-z

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