Abstract

The function of the 'origin recognition complex' (ORC) in eukaryotic cells is to select genomic sites where pre-replication complexes (pre-RCs) can be assembled. Subsequent activation of these pre-RCs results in bi-directional DNA replication that originates at or close to the ORC DNA binding sites. Recent results have revealed that one or more of the six ORC subunits is modified during the G1 to S-phase transition in such a way that ORC activity is inhibited until mitosis is complete and a nuclear membrane is assembled. In yeast, Cdk1/Clb phosphorylates ORC. In frog eggs, pre-RC assembly destabilizes ORC/chromatin sites, and ORC is eventually hyperphosphorylated and released. In mammals, the affinity of Orc1 for chromatin is selectively reduced during S-phase and restored during early G1-phase. Unbound Orc1 is ubiquitinated during S-phase and in some cases degraded. Thus, most, perhaps all, eukaryotes exhibit some manifestation of an 'ORC cycle' that restricts the ability of ORC to initiate pre-RC assembly to the early G1-phase of the cell cycle, making the 'ORC cycle' the premier step in determining when replication begins.