Sundin O, Varshavsky A
Cell. 1981 Sep;25(3):659-69. doi: 10.1016/0092-8674(81)90173-2.
When SV40-infected cells are placed into hypertonic medium, newly synthesized DNA accumulates as form C catenated dimers. These molecules consist of two supercoiled monomer circles of SV40 DNA interlocked by one or more topological inter-twinings and are seen as transiently labeled inter-mediates during normal replication. Form C catenated dimers represent pure segregation intermediates, replicative DNA structures in which DNA synthesis is complete but which still require topological separation of the two daughter circles. Hypertonic shock seems to block selectively a type II topoisomerase activity involved in disentangling the two circles. This is reflected in the fact that form C catenated dimers that accumulate during the block are highly intertwined with catenation linkage numbers up to C(L) = 20. While initiation of replication is also inhibited by hypertonic treatment, ongoing SV40 DNA synthesis is not affected, and replication is free to proceed from the earliest cairns structure through to form C catenated dimers. The block to segregation is rapidly and completely released by shifting the cells back to normal medium. A much slower recovery of DNA segregation takes place on prolonged incubation in hypertonic medium, perhaps because of some cellular homeostatic mechanism. The results of this work lead to a detailed view of the final stages of SV40 DNA replication.
当被SV40感染的细胞置于高渗培养基中时，新合成的DNA会以C型连环二聚体的形式积累。这些分子由两个SV40 DNA的超螺旋单体环组成，通过一个或多个拓扑缠绕相互连锁，在正常复制过程中被视为瞬时标记的中间体。C型连环二聚体代表纯分离中间体，即DNA合成已完成但仍需要两个子环进行拓扑分离的复制性DNA结构。高渗休克似乎选择性地阻断了参与解开两个环的II型拓扑异构酶活性。这体现在以下事实上：在阻断过程中积累的C型连环二聚体高度缠绕，连环连接数高达C(L)=20。虽然高渗处理也会抑制复制起始，但正在进行的SV40 DNA合成不受影响，复制可以从最早的 Cairns结构自由进行到形成C型连环二聚体。通过将细胞转移回正常培养基中，分离阻断会迅速且完全解除。在高渗培养基中长时间孵育后，DNA分离的恢复要慢得多，这可能是由于某种细胞稳态机制。这项工作的结果为SV40 DNA复制的最后阶段提供了详细的见解。
