A TRUNCATED FORM OF THE PHO80 CYCLIN REDIRECTS THE PHO85 KINASE TO DISRUPT VACUOLE INHERITANCE IN SACCHAROMYCES-CEREVISIAE

Partitioning of the vacuole during cell division in Saccharomyces cere visiae begins during early S phase and ends in late G2 phase before th e yeast nucleus migrates into the bud neck. We have isolated and chara cterized a new mutant, vac5-1, which is defective in vacuole segregati on. Cells with the vac5-1 mutation can form large buds without vacuole s. The VAC5 gene was cloned and is identical to PHO80. PHO80 encodes a cyclin which acts in a complex with a cdc-like kinase, PHO85, as a ne gative regulator of two transcription factors (PHO2 and PHO4) that gov ern the expression of metabolic phosphatases. The vacuole inheritance defect in vac5-1 cells is dependent on the presence of the Pho85 kinas e and its targets Pho4p and Pho2p. As with other alleles of PHO80, pho sphatase levels are elevated in vac5-1 mutants. A suppressor, the COOH -terminal half of the Gal11 transcription factor, rescues the vac5-1 p henotype of defective vacuole inheritance without altering the vac5-1 phenotype of elevated phosphatase levels. In addition, neither maximal nor minimal levels of expression of the inducible ''PHO'' system phos phatases causes a vacuole inheritance defect. Though vac5-1 is recessi ve, pho80 Delta or pho85 Delta strains do not show a defect in vacuole inheritance, suggesting that vac5-1 is not a complete loss-of-functio n allele. Sequence analysis shows that the vac5-1 allele encodes a tru ncated form of the Pho80 cyclin and overexpression of vac5-1 in pho80 Delta cells causes a vacuole inheritance defect. We conclude that the vac5-1 allele directs the Pho85 kinase to regulate, via transcription factors Pho4 and Pho2, genes that affect vacuole inheritance but which are not known to be under normal PHO pathway control.