Qiu Yi, Wang Shiwei, Chen Zhi, Guo Yajie, Song Yuan
Department of Microbiology, College of Biological Sciences, China Agricultural University, Beijing, PR China.
State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, Beijing, PR China.
PLoS One. 2016 Feb 22;11(2):e0149533. doi: 10.1371/journal.pone.0149533. eCollection 2016.
CRISPR-Cas systems, the small RNA-dependent immune systems, are widely distributed in prokaryotes. However, only a small proportion of CRISPR-Cas systems have been identified to be active in bacteria. In this work, a naturally active type I-E CRISPR-Cas system was found in Streptomyces avermitilis. The system shares many common genetic features with the type I-E system of Escherichia coli, and meanwhile shows unique characteristics. It not only degrades plasmid DNA with target protospacers, but also acquires new spacers from the target plasmid DNA. The naive features of spacer acquisition in the type I-E system of S. avermitilis were investigated and a completely conserved PAM 5'-AAG-3' was identified. Spacer acquisition displayed differential strand bias upstream and downstream of the priming spacer, and irregular integrations of new spacers were observed. In addition, introduction of this system into host conferred phage resistance to some extent. This study will give new insights into adaptation mechanism of the type I-E systems in vivo, and meanwhile provide theoretical foundation for applying this system on the genetic modification of S. avermitilis.
CRISPR-Cas系统是一种依赖小RNA的免疫系统，广泛分布于原核生物中。然而，只有一小部分CRISPR-Cas系统被鉴定在细菌中具有活性。在这项研究中，在阿维链霉菌中发现了一种天然活性的I-E型CRISPR-Cas系统。该系统与大肠杆菌的I-E型系统具有许多共同的遗传特征，同时也表现出独特的特性。它不仅能降解带有靶标原间隔序列的质粒DNA，还能从靶标质粒DNA中获取新的间隔序列。对阿维链霉菌I-E型系统中间隔序列获取的原始特征进行了研究，并鉴定出一个完全保守的PAM 5'-AAG-3'。间隔序列获取在起始间隔序列的上游和下游表现出不同的链偏向性，并且观察到新间隔序列的不规则整合。此外，将该系统导入宿主在一定程度上赋予了噬菌体抗性。这项研究将为I-E型系统在体内的适应机制提供新的见解，同时为将该系统应用于阿维链霉菌的基因改造提供理论基础。
