Department of Biochemistry, Indian Institute of Science, Bangalore, India.
Crit Rev Biochem Mol Biol. 2010 Apr;45(2):125-45. doi: 10.3109/10409231003628007.
DNA methyltransferases (MTases) are a group of enzymes that catalyze the methyl group transfer from S-adenosyl-L-methionine in a sequence-specific manner. Orthodox Type II DNA MTases usually recognize palindromic DNA sequences and add a methyl group to the target base (either adenine or cytosine) on both strands. However, there are a number of MTases that recognize asymmetric target sequences and differ in their subunit organization. In a bacterial cell, after each round of replication, the substrate for any MTase is hemimethylated DNA, and it therefore needs only a single methylation event to restore the fully methylated state. This is in consistent with the fact that most of the DNA MTases studied exist as monomers in solution. Multiple lines of evidence suggest that some DNA MTases function as dimers. Further, functional analysis of many restriction-modification systems showed the presence of more than one or fused MTase genes. It was proposed that presence of two MTases responsible for the recognition and methylation of asymmetric sequences would protect the nascent strands generated during DNA replication from cognate restriction endonuclease. In this review, MTases recognizing asymmetric sequences have been grouped into different subgroups based on their unique properties. Detailed characterization of these unusual MTases would help in better understanding of their specific biological roles and mechanisms of action. The rapid progress made by the genome sequencing of bacteria and archaea may accelerate the identification and study of species- and strain-specific MTases of host-adapted bacteria and their roles in pathogenic mechanisms.
DNA 甲基转移酶（MTases）是一组酶，它们以序列特异性的方式催化 S-腺苷-L-甲硫氨酸中的甲基基团转移。正统的 II 型 DNA MTases 通常识别回文 DNA 序列，并在两条链上的靶碱基（腺嘌呤或胞嘧啶）上添加一个甲基基团。然而，有许多 MTases 识别不对称靶序列，并且在亚基组织上有所不同。在细菌细胞中，每次复制后，任何 MTase 的底物都是半甲基化 DNA，因此只需一次甲基化事件即可恢复完全甲基化状态。这与大多数研究的 DNA MTases 在溶液中以单体形式存在的事实一致。多条证据表明，一些 DNA MTases 作为二聚体发挥作用。此外，对许多限制修饰系统的功能分析表明，存在一个以上或融合的 MTase 基因。有人提出，存在两个负责识别和甲基化不对称序列的 MTases 将保护在 DNA 复制过程中产生的新生链免受同源限制内切酶的影响。在这篇综述中，根据其独特特性，将识别不对称序列的 MTases 分为不同的亚组。对这些不寻常的 MTases 的详细特征描述将有助于更好地理解它们的特定生物学作用和作用机制。细菌和古细菌基因组测序的快速进展可能会加速鉴定和研究宿主适应细菌的种特异性和菌株特异性 MTases 及其在致病机制中的作用。
