Open AccessHeterogeneity in nuclear transport does not affect the timing of DNA synthesis in quiescent mammalian nuclei induced to replicate in Xenopus egg extracts
Author(s) -
Sun W.H.,
Hola M.,
Baldwin N.,
Pedley K.,
Brooks R.F.
Publication year - 2001
Publication title -
cell proliferation
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.647
H-Index - 74
eISSN - 1365-2184
pISSN - 0960-7722
DOI - 10.1046/j.1365-2184.2001.00196.x
Subject(s) - xenopus , biology , dna replication , dna synthesis , dna , nuclear dna , microbiology and biotechnology , nuclear transport , replication timing , control of chromosome duplication , replication factor c , eukaryotic dna replication , cell nucleus , genetics , gene , nucleus , mitochondrial dna
. Intact G 0 nuclei from quiescent mammalian cells initiate DNA synthesis asynchronously in Xenopus egg extracts, despite exposure to the same concentration of replication factors. This indicates that individual nuclei differ in their ability to respond to the inducers of DNA replication. Since the induction of DNA synthesis requires the accumulation of replication factors by active nuclear transport, any variation in the rate of transport among nuclei could contribute to the variability of DNA replication. Using the naturally fluorescent protein allophycocyanin (APC) coupled with the nuclear localization sequence (NLS) of SV40 T antigen, as a marker of nuclear uptake, we show here that individual G 0 nuclei differ in their rate of transport over a range of more than 20‐fold. Surprisingly, this variation has no direct influence on the timing or extent of DNA synthesis. Similar results were obtained by monitoring the uptake of nucleoplasmin, a nuclear protein present at high levels in egg extracts. These experiments show that the initiation of DNA synthesis is not driven merely by the accumulation of replication factors to some threshold concentration. Instead, some other explanation is needed to account for the timing of initiation.