Woodcock D M, Crowther P J, Simmons D L, Cooper I A
Biochim Biophys Acta. 1984 Dec 14;783(3):227-33. doi: 10.1016/0167-4781(84)90033-2.
We have determined the DNA renaturation kinetics for those DNA sequences of the Chinese hamster ovary (CHO-K1) cells in which enzymatic cytosine methylation occurred immediately after strand synthesis and for those in which methylation was delayed after strand synthesis. DNA sequences showing immediate or delayed methylation were found to be distributed throughout all repetition classes of the DNA of these cells, with a slight concentration of immediate methylation in moderately repetitive sequences and with delayed methylation being slightly over-represented in the highly repetitive fraction. However, DNA sequences showing both classes of methylation were represented equally in unique DNA sequences. We interpret these data to mean that the methylase acting near the replication forks (the 'immediate' methylase) is a relatively inefficient enzyme, missing some 20% of hemimethylated sites produced by DNA replication in these cells. We suggest that the methylase performing maintenance methylation at sites remote from the replication forks (the 'delayed' methylase) is simply a back-up enzyme for the first and that it has no true sequence specificity. The implications of this for the function(s) of DNA methylation in mammalian cells are discussed.
我们已经确定了中国仓鼠卵巢（CHO-K1）细胞中那些在链合成后立即发生酶促胞嘧啶甲基化的DNA序列以及那些在链合成后甲基化延迟的DNA序列的DNA复性动力学。发现显示立即或延迟甲基化的DNA序列分布在这些细胞DNA的所有重复类别中，在中度重复序列中立即甲基化略有集中，而在高度重复部分中延迟甲基化略有过度代表。然而，显示这两类甲基化的DNA序列在单一DNA序列中所占比例相同。我们将这些数据解释为意味着在复制叉附近起作用的甲基化酶（“立即”甲基化酶）是一种相对低效的酶，在这些细胞中遗漏了约20%由DNA复制产生的半甲基化位点。我们认为在远离复制叉的位点进行维持甲基化的甲基化酶（“延迟”甲基化酶）仅仅是第一种甲基化酶的备用酶，并且它没有真正的序列特异性。本文讨论了这对哺乳动物细胞中DNA甲基化功能的影响。
