ABSTRACT
In the budding yeast Saccharomyces cerevisiae, the cell division cycle and sporulation are mutually exclusive cell fates; glucose, which stimulates the cell division cycle, is a potent inhibitor of sporulation. Addition of moderate concentrations of glucose (0.5%) to sporulation medium did not inhibit transcription of two key activators of sporulation, IME1 and IME2, but did increase levels of Sic1p, a cyclin-dependent kinase inhibitor, resulting in a block to meiotic DNA replication. The effects of glucose on Sic1p levels and DNA replication required Grr1p, a component of the SCFGrr1p ubiquitin ligase. Sic1p is negatively regulated by Ime2p kinase, and several observations indicate that glucose inhibits meiotic DNA replication through SCFGrr1p-mediated destruction of this kinase. First, Ime2p was destabilized in the presence of glucose, and this turnover required Grr1p, a second component of SCFGrr1p, Cdc53p, and an SCFGrr1p-associated E2 enzyme, Cdc34p. Second, Ime2p-ubiquitin conjugates were detected under conditions of rapid Ime2p turnover, and conjugation of Ime2p to ubiquitin required GRR1. Third, a mutant form of Ime2p (Ime2ΔPEST), in which a putative Grr1p-interacting sequence was deleted, was more stable than wild-type Ime2p. Finally, expression of the IME2
ΔPEST
allele bypassed the block to meiotic DNA replication caused by 0.5% glucose. In addition, Grr1p is required for later events in sporulation independently of its role in Ime2p turnover.
The cyclin-dependent kinase inhibitor 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole induces nongenotoxic, DNA replication-independent apoptosis of normal and leukemic cells, regardless of their p53 status
