Involvement of RAD9-dependent damage checkpoint control in arrest of cell cycle,induction of cell death,and chromosome instability caused by defects in origin recognition complex in Saccharomyces cerevisiae

Perturbation of origin firing in chromosome replication is a possible cause of spontaneous chromosome instability in multireplicon organisms. Here, we show that chromosomal abnormalities, including aneuploidy and chromosome rearrangement, were significantly increased in yeast diploid cells with defects in the origin recognition complex. The cell cycle of orc1-4/orc1-4 temperature-sensitive mutant was arrested at the G₂/M boundary, after several rounds of cell division at the restrictive temperature. However, prolonged incubation of the mutant cells at 37°C led to abrogation of G₂ arrest, and simultaneously the cells started to lose viability. A sharp increase in chromosome instability followed the abrogation of G₂ arrest. In orc1-4/orc1-4 rad9Δ/rad9Δ diploid cells grown at 37°C, G₂ arrest and induction of cell death were suppressed, while chromosome instability was synergistically augmented. These findings indicated that DNA lesions caused by a defect in Orc1p function trigger the RAD9-dependent checkpoint control, which ensures genomic integrity either by stopping the cell cycle progress until lesion repair, or by inducing cell death when the lesion is not properly repaired. At semirestrictive temperatures, orc2-1/orc2-1 diploid cells demonstrated G₂ arrest and loss of cell viability, both of which require RAD9-dependent checkpoint control. However, chromosome instability was not induced in orc2-1/orc2-1 cells, even in the absence of the checkpoint control. These data suggest that once cells lose the damage checkpoint control, perturbation of origin firing can be tolerated by the cells. Furthermore, although a reduction in origin-firing capacity does not necessarily initiate chromosome instability, the Orc1p possesses a unique function, the loss of which induces instability in the chromosome.