The origin recognition complex marks a replication origin in the human TOP1 gene promoter

Area of Science:

• Genomics
• Molecular Biology
• Cell Biology

Background:

• The precise locations of the origin recognition complex (ORC) in mammalian genomes remain largely undetermined.
• Understanding ORC localization is crucial for deciphering DNA replication initiation mechanisms.

Purpose of the Study:

• To identify and characterize the DNA sequences associated with human ORC in vivo.
• To determine if identified ORC binding sites function as origins of DNA replication.

Main Methods:

• In vivo cross-linking and chromatin immunoprecipitation (ChIP) using antibodies specific for hOrc2.
• Nascent strand abundance assays to identify replication origins.
• Analysis of DNA sequences including CpG islands, matrix attachment regions, and satellite DNA.

Main Results:

• Antibodies against hOrc2 precipitated chromatin containing other ORC proteins, confirming in vivo complex formation.
• A specific ORC binding region was identified at the CpG island upstream of the human TOP1 gene.
• This ORC binding site was confirmed as an origin of bidirectional replication.
• ORC was also found near the TOM1 gene and on an alphoid satellite sequence in centromeric heterochromatin.
• The heterochromatic ORC site did not function as a replication initiation site.

Conclusions:

• Human ORC proteins form multisubunit complexes on chromatin in vivo.
• ORC binds to a replication origin upstream of the TOP1 gene.
• ORC binding to heterochromatin may serve functions beyond replication initiation, warranting further investigation.