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Published September 1981 | public
Journal Article

Arrest of segregation leads to accumulation of highly intertwined catenated dimers: Dissection of the final stages of SV40 DNA replication


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 intertwinings and are seen as transiently labeled intermediates 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.

Additional Information

© 1981 by MIT. Received 7 May 1981, Revised 17 June 1981. We thank Frederick Boyce for helpful discussions and Nancy Fritz for secretarial assistance. These studies were supported by a grant to A.V. from the National Institutes of Health. O.S. was supported by National Science Foundation predoctoral fellowship. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC. Section 1734 solely to indicate this fact.

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