Meisel A, Bickle T A, Krüger D H, Schroeder C
Institute of Virology, Humboldt University Medical School, Charité, Berlin, Germany.
Nature. 1992 Jan 30;355(6359):467-9. doi: 10.1038/355467a0.
Type III restriction/modification enzyme recognize short, non-palindromic sequences that can be methylated on only one strand, with the paradoxical consequence that during replication of what is in effect hemimethylated DNA totally unmodified sites arise. Why the unmodified sites are not subject to suicidal restriction was not clear. Here we show that restriction requires two unmodified recognition sites that can be separated by different distances but which must be in inverse orientation. All of the unmodified sites in newly replicated DNA are of course in the same orientation, which explains why they are not restricted. This result may be of relevance to other manifestations of anisotropy in double-stranded DNA, such as genetic imprinting.
III型限制/修饰酶识别短的、非回文序列，这些序列只能在一条链上被甲基化，其矛盾的结果是，在实际上是半甲基化DNA的复制过程中会出现完全未修饰的位点。为什么未修饰的位点不会受到自杀性限制尚不清楚。在这里，我们表明限制需要两个未修饰的识别位点，它们可以被不同的距离隔开，但必须是反向的。新复制的DNA中所有未修饰的位点当然是相同的方向，这就解释了为什么它们不会受到限制。这一结果可能与双链DNA中各向异性的其他表现形式有关，比如遗传印记。
