Loading of replicative helicases onto DNA is a key event during the initiation of chromosomal DNA replication. It takes place at specific chromosomal regions termed origins and is facilitated by the ORC protein complex. By resolving the cryo-EM structures of DNA-bound ORC, researchers from the Bleichert group (now at Yale) broaden our understanding of how DNA replication is initiated in animals.
Accurate replication of chromosomal DNA is essential for the survival and propagation of living organisms. Prior to cell division, many different proteins work together and duplicate genomes by semi-conservative replication so that copied chromosomes can be segregated into daughter cells. Genome integrity is sustained by highly efficient and accurate DNA replication exactly once per cell cycle. Failure to replicate DNA precisely can alter gene copy number and chromosome ploidy, which can give rise to genomic instability and a variety of human diseases.
Eukaryotic DNA replication initiation relies on the origin recognition complex (ORC), a DNA-binding ATPase that loads the Mcm2–7 replicative helicase onto replication origins. In yeast, origins are defined by a conserved consensus sequence that is recognized by ORC. By contrast, how replication origins are defined in animals (or metazoans) has remained unclear, but chromatin cues and local DNA structure are thought to help mediate the recognition of the origins. In a new paper, researchers reported cryo-electron microscopy (cryo-EM) structures of DNA-bound Drosophila ORC with and without the co-loader Cdc6.
These structures reveal that Orc1 and Orc4 constitute the primary DNA binding site in the ORC ring and cooperate with the winged-helix domains to stabilize DNA bending. A loop region near the catalytic Walker B motif of Orc1 directly contacts DNA, allosterically coupling DNA binding to ORC’s ATPase site.
Correlating structural and biochemical data showed that DNA sequence modulates DNA binding and remodeling by ORC, and that DNA bending promotes Mcm2–7 loading in vitro. Together, these findings explain the distinct DNA sequence-dependencies of metazoan and S. cerevisiae initiators in origin recognition and support a model in which DNA geometry and bendability contribute to Mcm2–7 loading site selection in metazoans.
References: Schmidt, J.M., Bleichert, F. Structural mechanism for replication origin binding and remodeling by a metazoan origin recognition complex and its co-loader Cdc6. Nat Commun 11, 4263 (2020). https://doi.org/10.1038/s41467-020-18067-7 link: https://www.nature.com/articles/s41467-020-18067-7