Clustered regularly-interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins protect bacteria and archaea from their viruses, and anti-CRISPRs (Acrs) are small virus-encoded proteins that inhibit CRISPR-Cas immunity. Over 80 families of Acrs have been described to date; however, only three of these subvert Type III CRISPR-Cas immunity. Type III systems employ a complex network of Cas and accessory nucleases to degrade viral nucleic acids. Here, we discover and characterize AcrIIIA1, the first Type III-A specific anti-CRISPR protein. We demonstrate that AcrIIIA1 binds to Csm2 within the Cas10-Csm effector complex and attenuates Cas10’s DNase activity and second messenger production. Additionally, AcrIIIA1 associates with fragmented t(m)RNAs (acrIIIA1-RNAs), and we show that they co-purify with the Cas10-Csm complex during phage infection. Although the precise role(s) of acrIIIA1-RNAs remain unclear, we found that they bind stably to RNase R, a host-encoded nuclease known to bolster immunity, and RNase R has the capacity to degrade them. Altogether, our results support a model in which AcrIIIA1 and its associated RNAs target both core Cas and accessory nucleases to provide robust protection against Type III CRISPR-Cas immunity.

中文翻译：

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AcrIIIA1 是一种靶向核心 Cas 和辅助核酸酶的蛋白-RNA 抗 CRISPR 复合物


成簇的规则间隔短回文重复序列 （CRISPR） 和 CRISPR 相关 （Cas） 蛋白可保护细菌和古细菌免受病毒侵害，而抗 CRISPR （Acrs） 是抑制 CRISPR-Cas 免疫的小病毒编码蛋白。迄今为止，已经描述了 80 多个 Acrs 家族;然而，其中只有 3 个颠覆了 III 型 CRISPR-Cas 免疫。III 型系统利用 Cas 和辅助核酸酶的复杂网络来降解病毒核酸。在这里，我们发现并表征了 AcrIIIA1，这是第一个 III-A 型特异性抗 CRISPR 蛋白。我们证明 AcrIIIA1 与 Cas10-Csm 效应复合物内的 Csm2 结合，并减弱 Cas10 的 DNase 活性和第二信使的产生。此外，AcrIIIA1 与片段化的 t（m）RNA （acrIIIA1-RNAs） 相关，我们表明它们在噬菌体感染过程中与 Cas10-Csm 复合物共纯化。尽管 acrIIIA1-RNA 的确切作用尚不清楚，但我们发现它们与 RNase R 稳定结合，RNase R 是一种已知可增强免疫力的宿主编码核酸酶，并且 RNase R 具有降解它们的能力。总而言之，我们的结果支持一个模型，其中 AcrIIIA1 及其相关 RNA 同时靶向核心 Cas 和辅助核酸酶，以提供针对 III 型 CRISPR-Cas 免疫的强大保护。