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Origin recognition complex

In molecular biology, origin recognition complex (ORC) is a multi-subunit DNA binding complex (6 subunits) that binds in all eukaryotes and archaea in an ATP-dependent manner to origins of replication. The subunits of this complex are encoded by the ORC1, ORC2, ORC3, ORC4, ORC5 and ORC6 genes. ORC is a central component for eukaryotic DNA replication, and remains bound to chromatin at replication origins throughout the cell cycle. ORC directs DNA replication throughout the genome and is required for its initiation. ORC and Noc3p bound at replication origins serve as the foundation for assembly of the pre-replication complex (pre-RC), which includes Cdc6, Tah11 (a.k.a. Cdt1), and the Mcm2-Mcm7 complex. Pre-RC assembly during G1 is required for replication licensing of chromosomes prior to DNA synthesis during S phase. Cell cycle-regulated phosphorylation of Orc2, Orc6, Cdc6, and MCM by the cyclin-dependent protein kinase Cdc28 regulates initiation of DNA replication, including blocking reinitiation in G2/M phase. The ORC is present throughout the cell cycle bound to replication origins, but is only active in late mitosis and early G1. In yeast, ORC also plays a role in the establishment of silencing at the mating-type loci Hidden MAT Left (HML) and Hidden MAT Right (HMR). ORC participates in the assembly of transcriptionally silent chromatin at HML and HMR by recruiting the Sir1 silencing protein to the HML and HMR silencers. Both Orc1 and Orc5 bind ATP, though only Orc1 has ATPase activity. The binding of ATP by Orc1 is required for ORC binding to DNA and is essential for cell viability. The ATPase activity of Orc1 is involved in formation of the pre-RC. ATP binding by Orc5 is crucial for the stability of ORC as a whole. Only the Orc1-5 subunits are required for origin binding; Orc6 is essential for maintenance of pre-RCs once formed. Interactions within ORC suggest that Orc2-3-6 may form a core complex. A 2020 report suggests that budding yeast ORC dimerizes in a cell cycle dependent manner to control licensing.

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Related concepts (11)
Cdc6
Cdc6, or cell division cycle 6, is a protein in eukaryotic cells. It is mainly studied in the budding yeast Saccharomyces cerevisiae (). It is an essential regulator of DNA replication and plays important roles in the activation and maintenance of the checkpoint mechanisms in the cell cycle that coordinate S phase and mitosis. It is part of the pre-replicative complex (pre-RC) and is required for loading minichromosome maintenance (MCM) proteins onto the DNA, an essential step in the initiation of DNA synthesis.
Pre-replication complex
A pre-replication complex (pre-RC) is a protein complex that forms at the origin of replication during the initiation step of DNA replication. Formation of the pre-RC is required for DNA replication to occur. Complete and faithful replication of the genome ensures that each daughter cell will carry the same genetic information as the parent cell. Accordingly, formation of the pre-RC is a very important part of the cell cycle. As organisms evolved and became increasingly more complex, so did their pre-RCs.
DNA replication factor CDT1
CDT1 (Chromatin licensing and DNA replication factor 1) is a protein that in humans is encoded by the CDT1 gene. It is a licensing factor that functions to limit DNA from replicating more than once per cell cycle. The protein encoded by this gene is a key licensing factor in the assembly of pre-replication complexes (pre-RC), which occurs during the G1 phase of the cell cycle. In the assembly of pre-RCs, origin recognition complexes (ORC1-6) recognize and bind to DNA replication origins.
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