Gómez-Eichelmann M C, Ramírez-Santos J
Departamento de Biología Molecular, Universidad Nacional Autónoma de México, México, D.F.
J Mol Evol. 1993 Jul;37(1):11-24. doi: 10.1007/BF00170457.
The frequency and distribution of methylated cytosine (5-MeC) at CCATGG (Dcm sites) in 49 E. coli DNA loci (207,530 bp) were determined. Principal observations of this analysis were: (1) Dcm frequency was higher than expected from random occurrence but lower than calculated with Markov chain analysis; (2) CCTGG sites were found more frequently in coding than in noncoding regions, while the opposite was true for CCAGG sites; (3) Dcm site distribution does not exhibit any identifiably regular pattern on the chromosome; (4) Dcm sites at oriC are probably not important for accurate initiation of DNA replication; (5) 5-MeC in codons was more frequently found in first than in second and third positions; (6) there are probably few genes in which the mutation rate is determined mainly by DNA methylation. It is proposed that the function of Dcm methylase is to protect chromosomal DNA from restriction-enzyme EcoRII. The Dcm methylation contribution to determine frequency of oligonucleotides, mutation rate, and recombination level, and thus evolution of the E. coli genome, could be interpreted as a consequence of the acquisition of this methylation.
测定了49个大肠杆菌DNA位点（207,530碱基对）中CCATGG（Dcm位点）处甲基化胞嘧啶（5 - MeC）的频率和分布。该分析的主要观察结果如下：（1）Dcm频率高于随机出现的预期，但低于马尔可夫链分析计算的值；（2）CCTGG位点在编码区比非编码区更频繁出现，而CCAGG位点则相反；（3）Dcm位点在染色体上的分布没有呈现出任何可识别的规律模式；（4）oriC处的Dcm位点可能对DNA复制的准确起始不重要；（5）密码子中的5 - MeC在第一位比在第二位和第三位更频繁出现；（6）可能很少有基因的突变率主要由DNA甲基化决定。有人提出，Dcm甲基化酶的功能是保护染色体DNA免受限制性内切酶EcoRII的作用。Dcm甲基化对确定寡核苷酸频率、突变率和重组水平以及大肠杆菌基因组进化的贡献，可以解释为这种甲基化获得的结果。
