Regulated expression of toxic BD fusions. Constitutive expression of a BD fused to Drosophila cyclin EI (CycEI) leads to loss of a functional BD fusion as yeast are passaged. Yeast were transformed with plasmids expressing BD-CycEI from the constitutive ADH1 promoter (ADH1p) or from the regulated MAL62 promoter (MAL62p), and then grown for the indicated number of passages on glucose media, which represses expression from MAL62p but not ADH1p. Yeast from each passage were mated with strains containing the empty AD vector or expressing AD fused to Rux or Cdi4, and the diploids were replicated onto —leu X-Gal indicator plates containing both galactose and maltose to induce expression of the AD fusion and the MAL62p-driven BD-CycEI. The strain in which BD-CycEI expression is controlled by ADH1p gradually loses its interaction phenotype with Rux and Cdi4 as cells are passaged. Correspondingly, the level of background activation by BD-CycEI is also lost. In contrast, the strain in which BD-CycEI expression is controlled by MAL62p maintains its interaction phenotype and background activation from passage to passage.
