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Mol. Cell. Biol., 08 1997, 4312-4321, Vol 17, No. 8
JR Wu and DM Gilbert
At a distinct point during G1 phase (the origin decision point [ODP]),
Chinese hamster ovary (CHO) cell nuclei experience a transition (origin
choice) that is required for specific recognition of the dihydrofolate
reductase (DHFR) origin locus by Xenopus egg extracts. We have investigated
the relationship between the ODP and progression of CHO cells through G1
phase. Selection of the DHFR origin at the ODP was rapidly inhibited by
treatment of early G1-phase cells with the protein kinase inhibitor
2-aminopurine (2-AP). Inhibition of the ODP required administration of 2-AP
at least 3 h prior to phosphorylation of the retinoblastoma tumor
suppressor protein (Rb) and the restriction point (R point). Cells deprived
of either serum or isoleucine from metaphase throughout early G1 phase
acquired the capacity to replicate in Xenopus egg extract (replication
licensing) and subsequently passed through the ODP on the same schedule as
cells cultured in complete growth medium. After growth arrest at the R
point with hypophosphorylated Rb protein, serum- or isoleucine-deprived
cells experienced a gradual loss of replication licensing. However,
recognition of the DHFR origin by Xenopus egg cytosol remained stable in
growth-arrested cells until the point at which all nuclei had lost the
capacity to initiate replication. These results provide evidence that the
ODP requires a mitogen-independent protein kinase that is activated after
replication licensing and prior to R-point control.
Copyright © 1997, American Society for Microbiology
The replication origin decision point is a mitogen-independent, 2- aminopurine-sensitive, G1-phase event that precedes restriction point control
Department of Biochemistry and Molecular Biology, SUNY Health Science Center, Syracuse, New York 13210, USA.
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