CRISPR-Cas adaptive immune systems provide prokaryotes with defense against viruses by degradation of specific invading nucleic acids. Despite advances in the biotechnological exploitation of select systems, multiple CRISPR-Cas types remain uncharacterized. Here, we investigated the previously uncharacterized type I-D interference complex and revealed that it is a genetic and structural hybrid with similarity to both type I and type III systems. Surprisingly, formation of the functional complex required internal in-frame translation of small subunits from within the large subunit gene. We further show that internal translation to generate small subunits is widespread across diverse type I-D, I-B, and I-C systems, which account for roughly one quarter of CRISPR-Cas systems. Our work reveals the unexpected expansion of protein coding potential from within single cas genes, which has important implications for understanding CRISPR-Cas function and evolution.

CRISPR-Cas适应性免疫系统通过降解特定的入侵核酸为原核生物提供病毒防御能力。尽管在选择系统的生物技术开发方面取得了进步，但多种CRISPR-Cas类型仍未表征。在这里，我们调查了以前未表征的ID干扰复合物，并发现它是与I型和III型系统相似的遗传和结构杂合体。令人惊讶地，功能复合物的形成需要大亚基基因内部的小亚基的内部框内翻译。我们进一步表明，内部翻译产生小亚基的现象广泛分布在各种ID，IB和IC系统中，约占CRISPR-Cas系统的四分之一。cas基因，对理解CRISPR-Cas功能和进化具有重要意义。