DOI：10.1038/nature14302 中国科研用户发表 作者：Asaf Levy
摘要 : 被称为CRISPR的细菌免疫系统依赖于从入侵的噬菌体基因组或质粒获取短序列，这些DNA被称为 “间隔区”。“间隔区”的获取过程会避免结合宿主DNA，但宿主DNA与噬菌体DNA是怎样被区分的此前却不清楚。
被称为CRISPR的细菌免疫系统依赖于从入侵的噬菌体基因组或质粒获取短序列，这些DNA被称为 “间隔区”。“间隔区”的获取过程会避免结合宿主DNA，但宿主DNA与噬菌体DNA是怎样被区分的此前却不清楚。Rotem Sorek及同事发现，“间隔区”DNA的形成需要依赖于复制的DNA双链断裂。宿主DNA中八聚 “Chi序列”的出现概率要大得多，该序列会降低RecBCD的核酸酶活性，从而导致更少的片段。另外，噬菌体基因组所含能导致断裂的复制叉也更多。
CRISPR–Cas (clustered, regularly interspaced short palindromic repeats coupled with CRISPR-associated proteins) is a bacterial immunity system that protects against invading phages or plasmids. In the process of CRISPR adaptation, short pieces of DNA (‘spacers’) are acquired from foreign elements and integrated into the CRISPR array. So far, it has remained a mystery how spacers are preferentially acquired from the foreign DNA while the self chromosome is avoided. Here we show that spacer acquisition is replication-dependent, and that DNA breaks formed at stalled replication forks promote spacer acquisition. Chromosomal hotspots of spacer acquisition were confined by Chi sites, which are sequence octamers highly enriched on the bacterial chromosome, suggesting that these sites limit spacer acquisition from self DNA. We further show that the avoidance of self is mediated by the RecBCD double-stranded DNA break repair complex. Our results suggest that, in Escherichia coli, acquisition of new spacers largely depends on RecBCD-mediated processing of double-stranded DNA breaks occurring primarily at replication forks, and that the preference for foreign DNA is achieved through the higher density of Chi sites on the self chromosome, in combination with the higher number of forks on the foreign DNA. This model explains the strong preference to acquire spacers both from high copy plasmids and from phages.
来源： Nature 浏览次数：1