Selective instability of Orc1 protein accounts for the absence of functional origin recognition complexes during the M-G₁ transition in mammals

To investigate the events leading to initiation of DNA replication in mammalian chromosomes, the time when hamster origin recognition complexes (ORCs) became functional was related to the time when Orc1, Orc2 and Mcm3 proteins became stably bound to hamster chromatin. Functional ORCs, defined as those able to initiate DNA replication, were absent during mitosis and early GI phase, and reappeared as cells progressed through G₁ phase. Immunoblotting analysis revealed that hamster Orc1 and Orc2 proteins were present in nuclei at equivalent concentrations throughout the cell cycle, but only Orc2 was stably bound to chromatin. Orc1 and Mcm3 were easily eluted from chromatin during mitosis and early G₁ phase, but became stably bound during mid-G₁ phase, concomitant with the appearance of a functional pre-replication complex at a hamster replication origin. Since hamster Ore proteins are closely related to their human and mouse homologs, the unexpected behavior of hamster Orc₁ provides a novel mechanism in mammals for delaying assembly of prereplication complexes until mitosis is complete and a nuclear structure has formed.