CRISPR-Cas systems are bacterial defense systems for fighting against invaders such as bacteriophages and mobile genetic elements. To escape destruction by these bacterial immune systems, phages have co-evolved multiple anti-CRISPR (Acr) proteins, which inhibit CRISPR-Cas function. Many acr genes form an operon with genes encoding transcriptional regulators, called anti-CRISPR-associated (Aca) proteins. Aca10 is the most recently discovered Aca family that is encoded within an operon containing acrIC7 and acrIC6 in Pseudomonas citronellolis. Here, we report the high-resolution crystal structure of an Aca10 protein to unveil the molecular basis of transcriptional repressor role of Aca10 in the acrIC7-acrIC6-aca10 operon. We identified that Aca10 forms a dimer in solution, which is critical for binding specific DNA. We also showed that Aca10 directly recognizes a 21 bp palindromic sequence in the promoter of the acr operon. Finally, we revealed that R44 of Aca10 is a critical residue involved in the DNA binding, which likely results in a high degree of DNA bending.

中文翻译：

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Aca10 抑制抗 CRISPR 操纵子的分子基础

CRISPR-Cas 系统是细菌防御系统，用于对抗入侵者，例如噬菌体和移动遗传元件。为了逃避这些细菌免疫系统的破坏，噬菌体共同进化出多种抗 CRISPR (Acr) 蛋白，这些蛋白可抑制 CRISPR-Cas 功能。许多 acr 基因与编码转录调节因子的基因形成一个操纵子，称为抗 CRISPR 相关 (Aca) 蛋白。Aca10 是最近发现的 Aca 家族，其编码在香茅假单胞菌中包含 acrIC7 和 acrIC6 的操纵子内。在这里，我们报告了 Aca10 蛋白的高分辨率晶体结构，以揭示 Aca10 在 acrIC7-acrIC6-aca10 操纵子中的转录抑制作用的分子基础。我们发现 Aca10 在溶液中形成二聚体，这对于结合特定的 DNA 至关重要。我们还表明，Aca10 直接识别 acr 操纵子启动子中的 21 bp 回文序列。最后，我们发现 Aca10 的 R44 是参与 DNA 结合的关键残基，这可能导致 DNA 高度弯曲。