SEGREGATION OF UNREPLICATED CHROMOSOMES IN SACCHAROMYCES-CEREVISIAE REVEALS A NOVEL G(1) M-PHASE CHECKPOINT/

Saccharomyces cerevisiae dbf4 and cdc7 cell cycle mutants block initia tion of DNA synthesis (i.e., are iDS mutants) at 37 degrees C and arre st the cell cycle with a 1C DNA content. Surprisingly, certain dbf4 an d cdc7 strains divide their chromatin at 37 degrees C. We found that t he activation of the Cdc28 mitotic protein kinase and the Dbf2 kinase occurred with the correct relative timing with respect to each other a nd the observed division of the unreplicated chromatin. Furthermore, t he division of unreplicated chromatin depended on a functional spindle . Therefore, the observed nuclear division resembled a normal mitosis, suggesting that S. cerevisiae commits to M phase in late G(1) indepen dently of S phase. Genetic analysis of dbf4 and rdc7 strains showed th at the ability to restrain mitosis during a late G(1) block depended o n the genetic background of the strain concerned, since the dbf4 and c dc7 alleles examined showed the expected mitotic restraint in other ba ckgrounds. This restraint was genetically dominant to lack of restrain t, indicating that an active arrest mechanism, or checkpoint, was invo lved. However, none of the previously described mitotic checkpoint pat hways were defective in the IDS strains that carry out mitosis without replicated DNA, therefore indicating that the checkpoint pathway that arrests mitosis in IDS mutants is novel. Thus, spontaneous strain dif ferences have revealed that S. cerevisiae commits itself to mitosis in late G(1) independently of entry into S phase and that a novel checkp oint mechanism can restrain mitosis if cells are blocked in late G(1). We refer to this as the G(1)/M-phase checkpoint since it acts in G(1) to restrain mitosis.