Cas13-containing type VI CRISPR-Cas systems specifically target RNA; however, the mechanism of spacer acquisition remains unclear. We have previously reported the association of reverse transcriptase–Cas1 (RT–Cas1) fusion proteins with certain types of VI-A systems. Here, we show that RT–Cas1 fusion proteins are also recruited by type VI-B systems in bacteria from gut microbiomes, constituting a VI-B1 variant system that includes a CorA-encoding locus in addition to the CRISPR array and the RT–Cas1/Cas2 adaptation module. We found that type VI RT-CRISPR systems were functional for spacer acquisition, CRISPR array processing and interference activity, demonstrating that adaptive immunity mediated by these systems can function independently of other in trans systems. We provide evidence that the RT associated with these systems enables spacer acquisition from RNA molecules. We also found that CorA encoded by type VI-B1 RT-associated systems can transport divalent metal ions and downregulate Cas13b-mediated RNA interference. These findings highlight the importance of RTs in RNA-targeting CRISPR-Cas systems, potentially enabling the integration of RNA-derived spacers into CRISPR arrays as a mechanism against RNA-based invaders in specific environments.

中文翻译：

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与逆转录酶-Cas1 融合蛋白相关的 VI 型 CRISPR-Cas 系统的适应性免疫


含 Cas13 的 VI 型 CRISPR-Cas 系统特异性靶向 RNA;然而，垫片获取的机制仍不清楚。我们之前已经报道了逆转录酶-Cas1 （RT-Cas1） 融合蛋白与某些类型的 VI-A 系统的关联。在这里，我们表明 RT-Cas1 融合蛋白也被肠道微生物组细菌中的 VI-B 型系统募集，构成了一个 VI-B1 变体系统，该系统除了 CRISPR 阵列和 RT-Cas1/Cas2 适应模块外，还包括一个 CorA 编码基因座。我们发现 VI TYPE RT-CRISPR 系统对间隔区采集、CRISPR 阵列处理和干扰活性具有功能，表明由这些系统介导的适应性免疫可以独立于反式系统中的其他系统发挥作用。我们提供的证据表明，与这些系统相关的 RT 能够从 RNA 分子中获得间隔区。我们还发现，由 VI-B1 型 RT 相关系统编码的 CorA 可以转运二价金属离子并下调 Cas13b 介导的 RNA 干扰。这些发现强调了 RT 在 RNA 靶向 CRISPR-Cas 系统中的重要性，有可能使 RNA 衍生的间隔区能够整合到 CRISPR 阵列中，作为在特定环境中对抗基于 RNA 的入侵者的机制。