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CpG methylation of DNA restricts prereplication complex assembly in Xenopus egg extracts
1Division of Biology, University of California, San Diego, La Jolla, California 92093, USA.
Molecular and Cellular Biology
|September 16, 2003
Summary
Origin recognition complex (ORC) binding to methylated DNA is inhibited, halting replication. However, MCM loading and replication initiation occur on both methylated and unmethylated DNA, suggesting a broader initiation mechanism in metazoans.
Area of Science:
- Molecular Biology
- Epigenetics
- DNA Replication
Background:
- CpG methylation of DNA can impede DNA replication.
- The Origin Recognition Complex (ORC) plays a crucial role in initiating DNA replication.
Purpose of the Study:
- To investigate the impact of CpG methylation on ORC binding and DNA replication initiation.
- To elucidate the role of MCM proteins in replication initiation on methylated DNA.
Main Methods:
- Xenopus egg replication system.
- Methylated plasmid DNA with varying CpG densities.
- Analysis of ORC binding and MCM loading.
- Identification of replication initiation sites.
Main Results:
- ORC binding is inhibited by CpG methylation but restored by inserting low-CpG-density DNA.
- MCM loading occurs on both weakly and highly methylated DNA over large regions (approx. 2 kb).
- Replication initiation is coincident with MCM loading, even at distal sites from ORC.
Conclusions:
- ORC binding is restricted to low-CpG DNA, acting as a "pseudo-origin".
- MCM loading and replication initiation can occur independently of direct ORC binding to highly methylated DNA.
- Metazoan replication initiation involves MCM loading and activation over extensive DNA regions, potentially distant from ORC.