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Title: PAM binding ensures orientational integration during Cas4-Cas1-Cas2 mediated CRISPR adaptation
Authors: Dhingra, Yukti
Keywords: Liên kết; PAM; tích hợp; CRISPR; trung gian; Cas4
Issue Date: 2023
Publisher: bioRxiv
Abstract: Adaptation in CRISPR-Cas systems immunizes bacteria and archaea against mobile genetic 10 elements. In many DNA-targeting systems, the Cas4-Cas1-Cas2 complex is required for selection 11 and processing of DNA segments containing PAM sequences, prior to integration of these 12 “prespacer” substrates as spacers in the CRISPR array. We determined cryo-EM structures of the 13 Cas4-Cas1-Cas2 adaptation complex from the type I-C system that encodes standalone Cas1 and 14 Cas4 proteins. The structures reveal how Cas4 specifically reads out bases within the PAM 15 sequence and how interactions with both Cas1 and Cas2 activate Cas4 endonuclease activity. The 16 Cas4-PAM interaction ensures tight binding between the adaptation complex and the prespacer, 17 significantly enhancing integration of the non-PAM end into the CRISPR array and ensuring 18 correct spacer orientation.
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