Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins provide bacteria with RNA-based adaptive immunity against phage infection. To counteract this defense mechanism, phages evolved anti-CRISPR (Acr) proteins that inactivate the CRISPR-Cas systems. AcrIIA1, encoded by Listeria monocytogenes prophages, is the most prevalent among the Acr proteins targeting type II-A CRISPR-Cas systems and has been used as a marker to identify other Acr proteins. Here, we report the crystal structure of AcrIIA1 and its RNA-binding affinity. AcrIIA1 forms a dimer with a novel two helical-domain architecture. The N-terminal domain of AcrIIA1 exhibits a helix-turn-helix motif similar to transcriptional factors. When overexpressed in Escherichia coli, AcrIIA1 associates with RNAs, suggesting that AcrIIA1 functions via nucleic acid recognition. Taken together, the unique structural and functional features of AcrIIA1 suggest its distinct mode of Acr activity, expanding the diversity of the inhibitory mechanisms employed by Acr proteins.

中文翻译：

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抗CRISPR蛋白AcrIIA1的晶体结构

簇状规则间隔的短回文重复序列（CRISPR）和CRISPR相关（Cas）蛋白为细菌提供了基于RNA的针对噬菌体感染的适应性免疫力。为了抵消这种防御机制，噬菌体进化出了使CRISPR-Cas系统失活的抗CRISPR（Acr）蛋白。由单核细胞增生李斯特菌原噬菌体编码的AcrIIA1是靶向II-A型CRISPR-Cas系统的Acr蛋白中最普遍的蛋白，已被用作鉴定其他Acr蛋白的标记。在这里，我们报告AcrIIA1的晶体结构及其RNA结合亲和力。AcrIIA1形成具有新颖的两个螺旋结构域结构的二聚体。AcrIIA1的N末端域显示出类似于转录因子的螺旋-转-螺旋基序。在大肠杆菌中过表达时，AcrIIA1与RNA关联，表明AcrIIA1通过核酸识别起作用。两者合计，AcrIIA1的独特结构和功能特征表明其独特的Acr活性模式，扩大了Acr蛋白采用的抑制机制的多样性。