Dynamics of activation of a galactose-inducible promoter in Saccharomyces cerevisiae

We have investigated the dynamics of accumulation of the Escherichia coli β-galactosidase (β-gal) under the control of a promoter containing the galactose-inducible upstream activating sequence (UAS_G) in single Saccharomyces cerevisiae cells. The accumulation of β-gal in single cells following the addition of the inducer, galactose, was determined using an in situ combined DNA and immunofluorescent stain in conjunction with flow cytometry. Two strains were studied, D603/2i, which has two copies of the galactose-inducible fusion gene integrated into its genome, and D603/pLGSD5, which carries a 2 μm-based plasmid containing the fusion gene. Flow cytometry results indicate that accumulation of β-gal within the first three hours following the addition of galactose is dependent on cell cycle position. Two proposed mechanisms explaining this observed behavior are (1) the cell-cycle-dependent synthesis of the fusion protein or (2) the unequal partitioning of the protein at cell division between mother and daughter cells.