CDC28 TYROSINE PHOSPHORYLATION AND THE MORPHOGENESIS CHECKPOINT IN BUDDING YEAST

A morphogenesis checkpoint in budding yeast delays nuclear division (a nd subsequent cell cycle progression) in cells that have failed to mak e a bud. We show that the ability of this checkpoint to delay nuclear division requires the SWE1 gene, encoding a protein kinase that inhibi ts the master cell cycle regulatory kinase Cdc28. The timing of nuclea r division in cells that cannot make a bud is exquisitely sensitive to the dosage of SWE1 and MIH1 genes, which control phosphorylation of C dc28 at tyrosine 19. In contrast, the timing of nuclear division in bu dded cells does not rely on Cdc28 phosphorylation, suggesting that the morphogenesis checkpoint somehow turns on this regulatory pathway. We show that SWE1 mRNA levels fluctuate during the cell cycle and are el evated in cells that cannot make a bud. However, regulation of SWE1 mR NA levels by the checkpoint is indirect, acting through a feedback loo p requiring Swe1 activity. Further, the checkpoint is capable of delay ing nuclear division even when SWE1 transcription is deregulated. We p ropose that the checkpoint delays nuclear division through posttransla tional regulation of Swe1 and that transcriptional feedback loops enha nce the efficacy of the checkpoint.