Bacteria use adaptive CRISPR-Cas immune mechanisms to protect from invasion by bacteriophages and other mobile genetic elements. In response, bacteriophages and mobile genetic elements have co-evolved anti-CRISPR proteins to inhibit the bacterial defense. We and others have previously shown that anti-CRISPR associated (Aca) proteins can regulate this anti-CRISPR counter-attack. Here, we report the first structure of an Aca protein, the Aca2 DNA-binding transcriptional autorepressor from Pectobacterium carotovorum bacteriophage ZF40, determined to 1.34 Å. Aca2 presents a conserved N-terminal helix-turn-helix DNA-binding domain and a previously uncharacterized C-terminal dimerization domain. Dimerization positions the Aca2 recognition helices for insertion into the major grooves of target DNA, supporting its role in regulating anti-CRISPRs. Furthermore, database comparisons identified uncharacterized Aca2 structural homologs in pathogenic bacteria, suggesting that Aca2 represents the first characterized member of a more widespread family of transcriptional regulators.

细菌使用适应性 CRISPR-Cas 免疫机制来保护免受噬菌体和其他可移动遗传元件的入侵。作为回应，噬菌体和移动遗传元件共同进化出抗 CRISPR 蛋白以抑制细菌防御。我们和其他人之前已经表明，抗 CRISPR 相关 (Aca) 蛋白可以调节这种抗 CRISPR 反击。在这里，我们报告了 Aca 蛋白的第一个结构，即来自Pectobacterium carotovorum的 Aca2 DNA 结合转录自抑制因子噬菌体 ZF40，测定为 1.34 Å。Aca2 提供了一个保守的 N 端螺旋-转角-螺旋 DNA 结合域和一个以前未表征的 C 端二聚化域。二聚化将 Aca2 识别螺旋定位为插入目标 DNA 的主要凹槽，支持其在调节抗 CRISPR 中的作用。此外，数据库比较确定了病原菌中未表征的 Aca2 结构同源物，这表明 Aca2 代表了更广泛的转录调节因子家族的第一个特征成员。