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Address correspondence to David M. Gilbert, Department of Biochemistry and Molecular Biology, State University of New York Upstate Medical University, 750 East Adams St., Syracuse, NY 13210. Tel.: (315) 464-8723. Fax: (315) 464-8750. E-mail: gilbertd{at}mail.upstate.edu
We have examined the dynamics of nuclear repositioning and the establishment of a replication timing program for the actively transcribed dihydrofolate reductase (DHFR) locus and the silent ß-globin gene locus in Chinese hamster ovary cells. The DHFR locus was internally localized and replicated early, whereas the ß-globin locus was localized adjacent to the nuclear periphery and replicated during the middle of S phase, coincident with replication of peripheral heterochromatin. Nuclei were prepared from cells synchronized at various times during early G1 phase and stimulated to enter S phase by introduction into Xenopus egg extracts, and the timing of DHFR and ß-globin replication was evaluated in vitro. With nuclei isolated 1 h after mitosis, neither locus was preferentially replicated before the other. However, with nuclei isolated 2 or 3 h after mitosis, there was a strong preference for replication of DHFR before ß-globin. Measurements of the distance of DHFR and ß-globin to the nuclear periphery revealed that the repositioning of the ß-globin locus adjacent to peripheral heterochromatin also took place between 1 and 2 h after mitosis. These results suggest that the CHO ß-globin locus acquires the replication timing program of peripheral heterochromatin upon association with the peripheral subnuclear compartment during early G1 phase.
Key Words: ß-globin; DNA replication; cell cycle; heterochromatin; nuclear organization
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