A detailed analysis is carried out on both published experimental results and new experiments for the methylation kinetics of two-site DNA substrates (with site separations between 100 and 800 bp) catalyzed by bacterial DNA adenine methyltransferase (Dam). A previously reported rate enhancement for the second methylation event (relative to that of the first methylation) is shown to result from elevated substrate specificity for singly methylated DNA over that of unmethylated DNA and not processive turnover of both sites by the same copy of Dam. An elementary model is suggested that cleanly fits the experimental data over a broad range of intersite separations. The model hypothesizes a looping mediated interference between competing unmethylated Dam sites on the same DNA strand.

中文翻译：

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DNA顺序修饰中的特异性与可加工性：DNA腺嘌呤甲基转移酶的研究

对已发表的实验结果和新实验均进行了详细的分析，以研究细菌DNA腺嘌呤甲基转移酶（Dam）催化的两个位点DNA底物（位点间隔在100到800 bp之间）的甲基化动力学。先前报道的第二次甲基化事件的速率增强（相对于第一次甲基化的速率）显示是由于单甲基化DNA的底物特异性高于未甲基化DNA的底物特异性，而不是同一拷贝的Dam导致两个位点的进行性转换。建议使用一个基本模型，该模型可以在广泛的站点间分离范围内完全拟合实验数据。该模型假设同一DNA链上竞争性未甲基化Dam位点之间存在环介导的干扰。