Bacteriophages encode anti-CRISPR suppressors to counteract the CRISPR/Cas immunity of their bacterial hosts, thus facilitating their survival and replication. Previous studies have shown that two phage-encoded anti-CRISPR proteins, AcrF1 and AcrF2, suppress the type I-F CRISPR/Cas system of Pseudomonas aeruginosa by preventing target DNA recognition by the Csy surveillance complex, but the precise underlying mechanism was unknown. Here we present the structure of AcrF1/2 bound to the Csy complex determined by cryo-EM single-particle reconstruction. By structural analysis, we found that AcrF1 inhibits target DNA recognition of the Csy complex by interfering with base pairing between the DNA target strand and crRNA spacer. In addition, multiple copies of AcrF1 bind to the Csy complex with different modes when working individually or cooperating with AcrF2, which might exclude target DNA binding through different mechanisms. Together with previous reports, we provide a comprehensive working scenario for the two anti-CRISPR suppressors, AcrF1 and AcrF2, which silence CRISPR/Cas immunity by targeting the Csy surveillance complex.

中文翻译：

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抗CRISPR病毒抑制剂AcrF1 / 2与Csy监测复合物的交替结合模式由冷冻EM结构揭示。

噬菌体编码抗CRISPR抑制剂来抵消细菌宿主的CRISPR / Cas免疫，从而促进其存活和复制。先前的研究表明，两种噬菌体编码的抗CRISPR蛋白AcrF1和AcrF2通过阻止Csy监视复合物识别靶DNA来抑制铜绿假单胞菌的IF CRISPR / Cas系统，但确切的潜在机制尚不清楚。在这里，我们介绍AcrF1 / 2绑定到Csy复杂由冷冻EM单粒子重建确定的结构。通过结构分析，我们发现AcrF1通过干扰DNA靶链与crRNA间隔子之间的碱基配对来抑制Csy复合物的靶DNA识别。此外，当单独工作或与AcrF2协同工作时，AcrF1的多个副本会以不同的模式结合到Csy复合体上，这可能会通过不同的机制排除靶DNA的结合。连同以前的报告，我们为两种抗CRISPR抑制剂AcrF1和AcrF2提供了全面的工作方案，它们通过靶向Csy监测复合物而沉默了CRISPR / Cas的免疫力。