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Vol. 9, Issue 5, 1163-1176, May 1998
and
*Department of Biochemistry, University of Mississippi Medical
Center, Jackson, Mississippi 39216; and
Somatic histone H1 reduces both the rate and extent of DNA
replication in Xenopus egg extract. We show here that H1
inhibits replication directly by reducing the number of replication
forks, but not the rate of fork progression, in Xenopus
sperm nuclei. Density substitution experiments demonstrate that those
forks that are active in H1 nuclei elongate to form large tracts of fully replicated DNA, indicating that inhibition is due to a reduction in the frequency of initiation and not the rate or extent of
elongation. The observation that H1 dramatically reduces the number of
replication foci in sperm nuclei supports this view. The establishment
of replication competent DNA in egg extract requires the assembly of
prereplication complexes (pre-RCs) on sperm chromatin. H1 reduces binding of the pre-RC proteins, XOrc2, XCdc6, and XMcm3, to chromatin. Replication competence can be restored in these nuclei, however, only
under conditions that promote the loss of H1 from chromatin and
licensing of the DNA. Thus, H1 inhibits replication in egg extract by
preventing the assembly of pre-RCs on sperm chromatin, thereby reducing
the frequency of initiation. These data raise the interesting
possibility that H1 plays a role in regulating replication origin use
during Xenopus development.
Wellcome/CRC
Institute, Cambridge CB2 1QR, England
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