PDF(Pubmed)
Abstract：
Type VI CRISPR-Cas systems are among the few CRISPR varieties that target exclusively RNA. The CRISPR RNA-guided, sequence-specific binding of target RNAs, such as phage transcripts, activates the type VI effector, Cas13. Once activated, Cas13 causes collateral RNA cleavage, which induces bacterial cell dormancy, thus protecting the host population from the phage spread. We show here that the principal form of collateral RNA degradation elicited by Leptotrichia shahii Cas13a expressed in Escherichia coli cells is the cleavage of anticodons in a subset of transfer RNAs (tRNAs) with uridine-rich anticodons. This tRNA cleavage is accompanied by inhibition of protein synthesis, thus providing defense from the phages. In addition, Cas13a-mediated tRNA cleavage indirectly activates the RNases of bacterial toxin-antitoxin modules cleaving messenger RNA, which could provide a backup defense. The mechanism of Cas13a-induced antiphage defense resembles that of bacterial anticodon nucleases, which is compatible with the hypothesis that type VI effectors evolved from an abortive infection module encompassing an anticodon nuclease.
摘要：
VI型CRISPR-Cas系统是少数仅靶向RNA的CRISPR变种之一。CRISPRRNA指导，靶RNA的序列特异性结合，比如噬菌体转录物，激活类型VI效应器，Cas13一旦激活,Cas13导致附带RNA裂解，诱导细菌细胞休眠，从而保护宿主群体免受噬菌体传播。我们在这里表明，在大肠杆菌细胞中表达的shahiiLeptotrichiaCas13a引起的附带RNA降解的主要形式是在具有富含尿苷的反密码子的转移RNA（tRNA）子集中切割反密码子。这种tRNA切割伴随着蛋白质合成的抑制，从而为噬菌体提供防御。此外,Cas13a介导的tRNA切割间接激活细菌毒素-抗毒素模块切割信使RNA的RNA酶，可以提供后备防御。Cas13a诱导的抗噬菌体防御机制类似于细菌反密码子核酸酶，这与以下假设相符：VI型效应子是从包含反密码子核酸酶的流产感染模块进化而来的。
