CRISPR adaptive immune systems protect bacteria from infections by deploying CRISPR RNA (crRNA)-guided enzymes to recognize and cut foreign nucleic acids. Type VI-A CRISPR–Cas systems include the Cas13a enzyme, an RNA-activated RNase capable of crRNA processing and single-stranded RNA degradation upon target-transcript binding. Here we present the 2.0-Å resolution crystal structure of a crRNA-bound Lachnospiraceae bacterium Cas13a (LbaCas13a), representing a recently discovered Cas13a enzyme subtype. This structure and accompanying biochemical experiments define the Cas13a catalytic residues that are directly responsible for crRNA maturation. In addition, the orientation of the foreign-derived target-RNA-specifying sequence in the protein interior explains the conformational gating of Cas13a nuclease activation. These results describe how Cas13a enzymes generate functional crRNAs and how catalytic activity is blocked before target-RNA recognition, with implications for both bacterial immunity and diagnostic applications.

中文翻译：

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靶向RNA的A裂解CRISPR–Cas13a酶的引导结合结构

CRISPR自适应免疫系统通过部署CRISPR RNA（crRNA）引导的酶来识别和切割外来核酸，从而保护细菌免受感染。VI-A型CRISPR-Cas系统包括Cas13a酶，一种RNA活化的RNase，能够在靶标与转录本结合后进行crRNA处理和单链RNA降解。在这里，我们介绍crRNA绑定的Lachnospiraceae细菌的2.0-Å分辨率晶体结构。Cas13a（LbaCas13a），代表最近发现的Cas13a酶亚型。这种结构和伴随的生化实验确定了直接导致crRNA成熟的Cas13a催化残基。此外，蛋白质内部的外源性靶标RNA指定序列的方向可以解释Cas13a核酸酶激活的构象门控。这些结果描述了Cas13a酶如何产生功能性crRNA，以及在识别靶RNA之前如何阻断催化活性，这对细菌免疫和诊断应用都有影响。