HISTONE H1 REDUCES THE FREQUENCY OF INITIATION IN XENOPUS EGG EXTRACTBY LIMITING THE ASSEMBLY OF PREREPLICATION COMPLEXES ON SPERM CHROMATIN

Somatic histone H1 reduces both the rate and extent of DNA replication in Xenopus egg extract. We show here that HI inhibits replication dir ectly by reducing the number of replication forks, but not the rate of fork progression, in Xenopus sperm nuclei. Density substitution exper iments demonstrate that those forks that are active in H1 nuclei elong ate to form large tracts of fully replicated DNA, indicating that inhi bition is due to a reduction in the frequency of initiation and not th e rate or extent of elongation. The observation that H1 dramatically r educes the number of replication foci in sperm nuclei supports this vi ew. The establishment of replication competent DNA in egg extract requ ires the assembly of prereplication complexes (pre-RCs) on sperm chrom atin. H1 reduces binding of the pre-RC proteins, XOrc2, XCdc6, and XMc m3, to chromatin. Replication competence can be restored in these nucl ei, however, only under conditions that promote the loss of H1 from ch romatin and licensing of the DNA. Thus, H1 inhibits replication in egg extract by preventing the assembly of pre-RCs on sperm chromatin, the reby reducing the frequency of initiation. These data raise the intere sting possibility that H1 plays a role in regulating replication origi n use during Xenopus development.