The replication timing program of the Chinese hamster beta-globin locus is established coincident with its repositioning near peripheral heterochromatin in early G1 phase.
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 beta-globin gene locus in Chinese hamster ovary cells. The DHFR locus was internally localized and replicated early, whereas the beta-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 beta-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 beta-globin. Measurements of the distance of DHFR and beta-globin to the nuclear periphery revealed that the repositioning of the beta-globin locus adjacent to peripheral heterochromatin also took place between 1 and 2 h after mitosis. These results suggest that the CHO beta-globin locus acquires the replication timing program of peripheral heterochromatin upon association with the peripheral subnuclear compartment during early G1 phase.
Li
F
F
Department of Biochemistry and Molecular Biology, State University of New York Upstate Medical University, Syracuse, NY 13210, USA.
Chen
J
J
Izumi
M
M
Butler
M C
MC
Keezer
S M
SM
Gilbert
D M
DM
D000818:Animals; D001973:Bromodeoxyuridine; D016466:CHO Cells; D002467:Cell Nucleus / Q000378:metabolism / Q000648:ultrastructure; D006224:Cricetinae; D004247:DNA / Q000096:biosynthesis*; D004261:DNA Replication / Q000502:physiology*; D016193:G1 Phase / Q000502:physiology*; D005914:Globins / Q000096:biosynthesis / Q000235:genetics*; D006570:Heterochromatin / Q000378:metabolism*; D017404:In Situ Hybridization, Fluorescence; D008938:Mitosis / Q000502:physiology; D016196:S Phase / Q000502:physiology; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics / Q000378:metabolism; D013997:Time Factors; D014981:Xenopus
D006570:Heterochromatin; D005914:Globins; D004247:DNA; D013762:Tetrahydrofolate Dehydrogenase; D001973:Bromodeoxyuridine