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Mol. Cell. Biol., Jun 1995, 2942-2954, Vol 15, No. 6
DM Gilbert, H Miyazawa and ML DePamphilis
Previous studies have shown that Xenopus egg extract can initiate DNA
replication in purified DNA molecules once the DNA is organized into a
pseudonucleus. DNA replication under these conditions is independent of DNA
sequence and begins at many sites distributed randomly throughout the
molecules. In contrast, DNA replication in the chromosomes of cultured
animal cells initiates at specific, heritable sites. Here we show that
Xenopus egg extract can initiate DNA replication at specific sites in
mammalian chromosomes, but only when the DNA is presented in the form of an
intact nucleus. Initiation of DNA synthesis in nuclei isolated from
G1-phase Chinese hamster ovary cells was distinguished from continuation of
DNA synthesis at preformed replication forks in S- phase nuclei by a delay
that preceded DNA synthesis, a dependence on soluble Xenopus egg factors,
sensitivity to a protein kinase inhibitor, and complete labeling of nascent
DNA chains. Initiation sites for DNA replication were mapped downstream of
the amplified dihydrofolate reductase gene region by hybridizing newly
replicated DNA to unique probes and by hybridizing Okazaki fragments to the
two individual strands of unique probes. When G1-phase nuclei were prepared
by methods that preserved the integrity of the nuclear membrane, Xenopus
egg extract initiated replication specifically at or near the origin of
bidirectional replication utilized by hamster cells (dihydrofolate
reductase ori-beta). However, when nuclei were prepared by methods that
altered nuclear morphology and damaged the nuclear membrane, preference for
initiation at ori-beta was significantly reduced or eliminated.
Furthermore, site-specific initiation was not observed with bare DNA
substrates, and Xenopus eggs or egg extracts replicated prokaryotic DNA or
hamster DNA that did not contain a replication origin as efficiently as
hamster DNA containing ori-beta. We conclude that initiation sites for DNA
replication in mammalian cells are established prior to S phase by some
component of nuclear structure and that these sites can be activated by
soluble factors in Xenopus eggs.
Copyright © 1995, American Society for Microbiology
Site-specific initiation of DNA replication in Xenopus egg extract requires nuclear structure
Roche Institute of Molecular Biology, Roche Research Center, Nutley, New Jersey 07110-1199, USA.
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