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Published August 1980 | public
Journal Article

Terminal stages of SV40 DNA replication proceed via multiply intertwined catenated dimers


We have identified a new class of SV40 replicative intermediates which consists of a least 20 discrete DNA species. All members of this class are catenated dimers, two circular molecules of SV40 duplex DNA linked topologically by one or more intertwining events. Most of these molecules are linked by several intertwining events, and the range of linkage states observed runs from L = 1 to L = 10. A catenated dimer with a given linkage state is assigned to one of three distinct families (A, B or C) depending on the open or covalently closed nature of its two circular components: in form A catenated dimers, both circles are nicked or gapped; in form B, one of the circles is supercoiled; and in form C, both circles are supercoiled. Members of all three of these families are found in SV40 chromatin pulse-labeled with ³H-thymidine, and together they comprise 10–20% of the total replicative form SV40 DNA, appearing as a discrete series of electrophoretically resolved bands superimposed upon a continuous smear of growing cairns structures. The distribution of linkage states varies between the families, A being the most intertwined and C the least intertwined. Upon a chase with cold thymidine, label is lost rapidly from all these catenated DNA species. We suggest that the sequence A → B → C → mature monomeric supercoiled SV40 DNA represents the final stages of SV40 replication, and that a special enzyme activity exists in vivo to uncatenate the SV40 daughter chromosomes.

Additional Information

© 1980 by MIT. Received 9 April 1980, Revised 20 May 1980. We thank Erika Hartweig and Jennie Fostei for their generous help with electron microscopy, Louis Levinger for advice on preparing the illustrations and Judy Currier for typing the manuscript. These studies were supported by grants to A.V. from the NIH and the Whittaker Health Sciences Fund. O.S. is supported by an NSF 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 U.S.C. Section 1734 solely to indicate this fact.

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