Origin recognition complex

Proteins

This process was initiated by the loading of Mcm2-7 onto the chromatid by the ORC and associated proteins ]] The following proteins are present in the ORC:

Archaea feature a simplified version of the ORC, Mcm, and as a consequence the combined pre-RC. Instead of using six different mcm proteins to form a pseudo-symmetrical heterohexamer, all six subunits in the archaeal MCM are the same. They usually have multiple proteins that are homologous to both Cdc6 and Orc1, some of which perform the function of both. Unlike eukaryotic Orc, they do not always form a complex. In fact, they have divergent complex structures when these do form. Sulfolobus islandicus also uses a Cdt1 homologue to recognize one of its replication origins.

Autonomously replicating sequences

Budding yeast

Autonomously Replicating Sequences (ARS), first discovered in budding yeast, are integral to the success of the ORC. These 100-200bp sequences facilitate replication activity during S phase. ARSs can be placed at any novel location of the chromosomes of budding yeast and will facilitate replication from those sites. A highly conserved sequence of 11bp (known as the A element) is thought to be essential for origin function in budding yeast. The ORC was originally identified by its ability to bind to the A element of the ARS in budding yeast.

Animals

Animal cells contain a much more cryptic version of an ARS, with no conserved sequences found as of yet. Here, replication origins gather into bundles called replicon clusters. Each cluster's replicons are similar in length, but individual clusters have replicons of varying length. These replicons all have similar basic residues to which the ORC binds, which in many ways mimic the conserved 11bp A element. All of these clusters are simultaneously activated during S phase.

Role in pre-RC assembly

The ORC is essential for the loading of MCM complexes (Pre-RC) onto DNA. This process is dependent on the ORC, Noc3, Cdc6, and Cdt1 – involving several ATP controlled recruiting events. First, the ORC, Noc3p and Cdc6 form a complex on origin DNA (marked by ARS type regions). New ORC/Noc3/Cdc6 complexes then recruit Cdt1/Mcm2-7 molecules to the site. Once this massive ORC/Noc3/Cdc6/Cdt1/Mcm2-7 complex is formed, the ORC/Noc3/Cdc6/Cdt1 molecules work together to load Mcm2-7 onto the DNA itself by hydrolysis of ATP by Cdc6. Cdc6's phosphorylative activity is dependent on both the ORC and origin DNA. This leads to Cdt1 having decreased stability on the DNA and falling off of the complex leading to Mcm2-7 loading on to the DNA. The structure of the ORC, MCM, as well as the intermediate OCCM complex has been resolved.

Origin binding activity

Although the ORC is composed of six discrete subunits, only one of these has been found to be significant - ORC1. In vivo studies have shown that Lys-263 and Arg-367 are the basic residues responsible for faithful ORC loading. These molecules represent the above-mentioned ARS. ORC1 interacts with ATP and these basic residues in order to bind the ORC to origin DNA. It has been established that this occurs far before replication, and that the ORC itself is already bound to Origin DNA by the time any Mcm2-7 loading occurs. When Mcm2-7 is first loaded it completely encircles the DNA and helicase activity is inhibited. In S phase, the Mcm2-7 complex interacts with helicase cofactors Cdc45 and GINS to isolate a single DNA strand, unwind the origin, and begin replication down the chromosome. In order to have bidirectional replication, this process happens twice at an origin. Both loading events are mediated by one ORC via an identical process as the first.

References

References

1. {{MeshName. Origin+Recognition+Complex
2. (1997). "Initiation of DNA replication in eukaryotic cells". Annual Review of Cell and Developmental Biology.
3. (2007). "Multiple Functions of the Origin Recognition Complex".
4. (December 2007). "Yeast two-hybrid analysis of the origin recognition complex of Saccharomyces cerevisiae: interaction between subunits and identification of binding proteins". FEMS Yeast Research.
5. (May 1992). "ATP-dependent recognition of eukaryotic origins of DNA replication by a multiprotein complex". Nature.
6. (November 1995). "The multidomain structure of Orc1p reveals similarity to regulators of DNA replication and transcriptional silencing". Cell.
7. (June 2006). "Cell cycle execution point analysis of ORC function and characterization of the checkpoint response to ORC inactivation in Saccharomyces cerevisiae". Genes to Cells.
8. (June 2002). "Noc3p, a bHLH Protein, Plays an Integral Role in the Initiation of DNA Replication in Budding Yeast". Cell.
9. (March 1995). "The origin recognition complex interacts with a bipartite DNA binding site within yeast replicators". Proceedings of the National Academy of Sciences of the United States of America.
10. (June 1995). "Initiation complex assembly at budding yeast replication origins begins with the recognition of a bipartite sequence by limiting amounts of the initiator, ORC". The EMBO Journal.
11. (November 2005). "ATPase-dependent cooperative binding of ORC and Cdc6 to origin DNA". Nature Structural & Molecular Biology.
12. (2000). "Regulation of chromosome replication". Annual Review of Biochemistry.
13. (2002). "DNA replication in eukaryotic cells". Annual Review of Biochemistry.
14. (February 2005). "Origin recognition and the chromosome cycle". FEBS Letters.
15. (September 2001). "Binding of cyclin-dependent kinases to ORC and Cdc6p regulates the chromosome replication cycle". Proceedings of the National Academy of Sciences of the United States of America.
16. (June 2001). "Cyclin-dependent kinases prevent DNA re-replication through multiple mechanisms". Nature.
17. (August 2005). "Disruption of mechanisms that prevent rereplication triggers a DNA damage response". Molecular and Cellular Biology.
18. (May 1996). "Role of interactions between the origin recognition complex and SIR1 in transcriptional silencing". Nature.
19. (June 1997). "The origin recognition complex, SIR1, and the S phase requirement for silencing". Science.
20. (February 1997). "Coordinate binding of ATP and origin DNA regulates the ATPase activity of the origin recognition complex". Cell.
21. (July 2001). "ATP bound to the origin recognition complex is important for preRC formation". Proceedings of the National Academy of Sciences of the United States of America.
22. (December 2004). "ATP hydrolysis by ORC catalyzes reiterative Mcm2-7 assembly at a defined origin of replication". Molecular Cell.
23. (January 2006). "Sequential ATP hydrolysis by Cdc6 and ORC directs loading of the Mcm2-7 helicase". Molecular Cell.
24. (November 2006). "An essential role for Orc6 in DNA replication through maintenance of pre-replicative complexes". The EMBO Journal.
25. (July 2021). "Replication initiation: Implications in genome integrity". DNA Repair.
26. (March 2020). "An Essential and Cell-Cycle-Dependent ORC Dimerization Cycle Regulates Eukaryotic Chromosomal DNA Replication". Cell Reports.
27. (January 2017). "Diversity of DNA Replication in the Archaea". Genes.
28. Morgan, David. (2007). "The Cell Cycle: Principles of Control". Primers in Biology.
29. (May 2013). "An ORC/Cdc6/MCM2-7 complex is formed in a multistep reaction to serve as a platform for MCM double-hexamer assembly". Molecular Cell.
30. (January 2006). "Sequential ATP hydrolysis by Cdc6 and ORC directs loading of the Mcm2-7 helicase". Molecular Cell.
31. (November 2005). "ATPase-dependent cooperative binding of ORC and Cdc6 to origin DNA". Nature Structural & Molecular Biology.
32. (March 2017). "Structural basis of Mcm2-7 replicative helicase loading by ORC-Cdc6 and Cdt1". Nature Structural & Molecular Biology.
33. (October 2015). "Specific binding of eukaryotic ORC to DNA replication origins depends on highly conserved basic residues". Scientific Reports.
34. (April 2015). "Single-Molecule Visualization of MCM2-7 DNA Loading: Seeing Is Believing". Cell.