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Updated: Jul 21, 2026

Direct Restart of a Replication Fork Stalled by a Head-On RNA Polymerase
Published on: April 29, 2010
Initiation of eukaryotic DNA replication: origin unwinding and sequential chromatin association of Cdc45, RPA, and
1Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA. johannes_walter@hms.harvard.edu
Abstract:
We report that a plasmid replicating in Xenopus egg extracts becomes negatively supercoiled during replication initiation. Supercoiling requires the initiation factor Cdc45, as well as the single-stranded DNA-binding protein RPA, and therefore likely represents origin unwinding. When unwinding is prevented, Cdc45 binds to chromatin whereas DNA polymerase alpha does not, indicating that Cdc45, RPA, and DNA polymerase alpha bind chromatin sequentially at the G1/S transition. Whereas the extent of origin unwinding is normally limited, it increases dramatically when DNA polymerase alpha is inhibited, indicating that the helicase that unwinds DNA during initiation can become uncoupled from the replication fork. We discuss the implications of these results for the location of replication start sites relative to the prereplication complex.
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