CRISPR-Cas immune systems in bacteria and archaea record prior infections as spacers within each system's CRISPR arrays. Spacers are normally derived from invasive genetic material and direct the immune system to complementary targets as part of future infections. However, not all spacers appear to be derived from foreign genetic material and instead can originate from the host genome. Their presence poses a paradox, as self-targeting spacers would be expected to induce an autoimmune response and cell death. In this review, we discuss the known frequency of self-targeting spacers in natural CRISPR-Cas systems, how these spacers can be incorporated into CRISPR arrays, and how the host can evade lethal attack. We also discuss how self-targeting spacers can become the basis for alternative functions performed by CRISPR-Cas systems that extend beyond adaptive immunity. Overall, the acquisition of genome-targeting spacers poses a substantial risk but can aid in the host's evolution and potentially lead to or support new functionalities.

中文翻译：

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CRISPR-Cas系统和自定位间隔子的悖论。

细菌和古细菌中的CRISPR-Cas免疫系统将以前的感染记录为每个系统的CRISPR阵列中的间隔子。间隔物通常来自侵入性遗传物质，并将免疫系统引导至互补靶标，作为未来感染的一部分。但是，并非所有间隔子似乎都来自外来遗传物质，而是可以源自宿主基因组。它们的存在构成一个悖论，因为自靶向间隔子会诱导自身免疫反应和细胞死亡。在这篇综述中，我们讨论了自然CRISPR-Cas系统中自靶向间隔子的已知频率，如何将这些间隔子整合到CRISPR阵列中以及宿主如何规避致命攻击。我们还将讨论自我定位的间隔子如何成为CRISPR-Cas系统执行的超越自适应免疫的替代功能的基础。总体而言，以基因组为靶标的间隔区的获得带来了巨大的风险，但可以帮助宿主的进化，并可能导致或支持新的功能。