Cooney C A, Eykholt R L, Bradbury E M
Department of Biological Chemistry, School of Medicine, University of California, Davis 95616.
J Mol Biol. 1988 Dec 20;204(4):889-901. doi: 10.1016/0022-2836(88)90049-6.
In Physarum polycephalum, the ribosomal DNA is found as 60,000 base-pair palindromes. Each rDNA has four symmetrically arranged replication origins flanked by ribosomal RNA genes. A particular sequence, the putative replication origin, is repeated at the approximate position of each origin and nowhere else in the molecule. On a typical rDNA molecule, only one origin is active per replication cycle. We show that both the level and co-ordination of methylation result in asymmetrically methylated rDNA molecules that are particularly hypomethylated at one of their four putative replication origins. This pattern of methylation on a typical rDNA molecule is consistent with a model where hypomethylation is a determinant of origin activity.
在多头绒泡菌中，核糖体DNA以60,000个碱基对的回文形式存在。每个核糖体DNA有四个对称排列的复制起点，两侧是核糖体RNA基因。一个特定序列，即假定的复制起点，在每个起点的大致位置重复出现，而在分子的其他位置则没有。在一个典型的核糖体DNA分子上，每个复制周期只有一个起点是活跃的。我们发现，甲基化的水平和协调性都会导致核糖体DNA分子发生不对称甲基化，在其四个假定的复制起点之一处尤其低甲基化。典型核糖体DNA分子上的这种甲基化模式与一种模型相符，即低甲基化是起点活性的一个决定因素。
