Raleigh E A
New England Biolabs, Beverly, Massachusetts 01915.
Mol Microbiol. 1992 May;6(9):1079-86. doi: 10.1111/j.1365-2958.1992.tb01546.x.
Many natural DNA sequences are restricted in Escherichia coli K-12, not only by the classic Type I restriction system EcoK, but also by one of three modification-specific restriction systems found in K-12. The McrBC system is the best studied of these. We infer from the base composition of the mcrBC genes that they were imported from an evolutionarily distant source. The genes are located in a hypervariable cluster of restriction genes that may play a significant role in generation of species identity in enteric bacteria. Restriction activity requires the products of two genes for activity both in vivo and in vitro. The mcrB gene elaborates two protein products, only one of which is required for activity in vitro, but both of which contain a conserved amino acid sequence motif identified as a possible GTP-binding site. The mcrC gene product contains a leucine heptad repeat that could play a role in protein-protein interactions. McrBC activity in vivo and in vitro depends on the presence of modified cytosine in a specific sequence context; three different modifications are recognized. The in vitro activity of this novel multi-subunit restriction enzyme displays an absolute requirement for GTP as a cofactor.
许多天然DNA序列在大肠杆菌K-12中受到限制，不仅受到经典的I型限制系统EcoK的限制，还受到K-12中发现的三种修饰特异性限制系统之一的限制。McrBC系统是其中研究得最深入的。我们从mcrBC基因的碱基组成推断，它们是从进化上遥远的来源导入的。这些基因位于一个高度可变的限制基因簇中，该基因簇可能在肠道细菌物种特异性的产生中起重要作用。限制活性在体内和体外都需要两个基因的产物。mcrB基因产生两种蛋白质产物，其中只有一种在体外活性中是必需的，但两种都含有一个保守的氨基酸序列基序，被鉴定为可能的GTP结合位点。mcrC基因产物包含一个亮氨酸七肽重复序列，可能在蛋白质-蛋白质相互作用中起作用。McrBC在体内和体外的活性取决于特定序列背景下修饰胞嘧啶的存在；可识别三种不同的修饰。这种新型多亚基限制酶的体外活性绝对需要GTP作为辅因子。
