CRISPR-Cas systems are prokaryotic immune systems against invading nucleic acids. Type I CRISPR-Cas systems employ highly diverse, multi-subunit surveillance Cascade complexes that facilitate duplex formation between crRNA and complementary target DNA for R-loop formation, retention, and DNA degradation by the subsequently recruited nuclease Cas3. Typically, the large subunit recognizes bona fide targets through the PAM (protospacer adjacent motif), and the small subunit guides the non-target DNA strand. Here, we present the Apo- and target-DNA-bound structures of the I-Fv (type I-F variant) Cascade lacking the small and large subunits. Large and small subunits are functionally replaced by the 5′ terminal crRNA cap Cas5fv and the backbone protein Cas7fv, respectively. Cas5fv facilitates PAM recognition from the DNA major groove site, in contrast to all other described type I systems. Comparison of the type I-Fv Cascade with an anti-CRISPR protein-bound I-F Cascade reveals that the type I-Fv structure differs substantially at known anti-CRISPR protein target sites and might therefore be resistant to viral Cascade interception.

CRISPR-Cas系统是针对入侵核酸的原核免疫系统。I型CRISPR-Cas系统采用高度多样化的多亚基监视级联复合体，可促进crRNA与互补靶DNA之间的双链体形成，从而通过随后募集的核酸酶Cas3进行R环形成，保留和DNA降解。通常，大的亚基通过PAM（与原间隔物相邻的基序）识别真正的靶标，而小的亚基则引导非靶标DNA链。在这里，我们介绍缺乏小亚基和大亚基的I-Fv（IF型变体）级联的Apo-和靶DNA结合结构。大亚基和小亚基在功能上分别被5'末端crRNA帽Cas5fv和骨架蛋白Cas7fv取代。Cas5fv有助于从DNA大沟部位识别PAM，与所有其他描述的I型系统相反。将I-Fv级联与结合抗CRISPR蛋白的IF级联进行比较表明，I-Fv型结构在已知的抗CRISPR蛋白靶位点上存在显着差异，因此可能对病毒级联拦截具有抗性。