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© The Rockefeller University Press,
0021-9525/2001/7/283 $5.00
The Journal of Cell Biology, Volume 154, Number 2, July 23, 2001 283-292
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The replication timing program of the Chinese hamster ß-globin locus is established coincident with its repositioning near peripheral heterochromatin in early G1 phase
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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