Bernheim Aude
Synthetic Biology Group, Institut Pasteur, 25-28 rue Dr. Roux, 75015 Paris, France - Microbial Evolutionary Genomics, Institut Pasteur, 25-28 rue Dr Roux, 75015 Paris, France - AgroParisTech, 75005 Paris, France.
Biol Aujourdhui. 2017;211(4):255-264. doi: 10.1051/jbio/2018005. Epub 2018 Jun 29.
CRISPR-Cas (Cluster of Regularly Interspaced Short Palindromic Repeats) systems confer bacteria and archaea an adaptative immunity against phages and other invading genetic elements playing an important role in bacterial evolution. However, despite the protection they generate and high rate of horizontal transfer, less than 50% of bacterial genomes harbor a CRISPR-Cas system. As a comparison, 90% of archaea encode a CRISPR-Cas system and a bacterial genome codes for two restriction modification systems on average. This review describes CRISPR-Cas systems distribution in bacterial genomes and then details the different hypotheses put forward to explain the relative scarcity of CRISPR-Cas systems. More specifically, phage escape mechanisms, ecological factors such as phage diversity and abundance and intrinsic costs, such as maintenance or autoimmunity, are discussed. Overall, a better understanding of the downsides of encoding CRISPR-Cas systems is essential to explain their evolutionary dynamics and their relative success in different environments and clades.
CRISPR-Cas（规律成簇的间隔短回文重复序列）系统赋予细菌和古菌针对噬菌体及其他入侵遗传元件的适应性免疫，在细菌进化中发挥着重要作用。然而，尽管它们能提供保护且水平转移率很高，但不到50%的细菌基因组含有CRISPR-Cas系统。相比之下，90%的古菌编码CRISPR-Cas系统，并且细菌基因组平均编码两个限制修饰系统。本综述描述了CRISPR-Cas系统在细菌基因组中的分布，然后详细阐述了为解释CRISPR-Cas系统相对稀缺性而提出的不同假说。更具体地说，讨论了噬菌体逃逸机制、诸如噬菌体多样性和丰度等生态因素以及诸如维持或自身免疫等内在成本。总体而言，更好地理解编码CRISPR-Cas系统的不利之处对于解释其进化动态以及它们在不同环境和进化枝中的相对成功至关重要。
