Figure 9.

NuA4 action is necessary to allow Pho4 binding to the PHO5 promoter, but is dispensable upon overexpression of Pho4. (A) Pho4 is enriched at the PHO5 promoter in wild-type but not esa1 mutant cells under inducing conditions. ChIP experiment using anti-Pho4 and preimmune sera with wild-type or esa1 ts cells grown in phosphate-rich (+Pi) media at 25°C or placed for 3 h in phosphate-free (−Pi) media at 37°C. IP ratios were calculated by real-time PCR reactions and presented as in Figure 8B using preimmune serum signals as subtracted backgrounds. (B) A similar ChIP experiment was performed with wild-type and mutant cells incubated for 6 h under inducing conditions at 37°C. Under these conditions, the presence of Pho4 at the PHO5 promoter is 10-fold higher in wild type than in mutant cells. (C) Overexpression of Pho4 bypasses both Pho2 and NuA4 requirements for PHO5 activation. esa1 ts mutant cells were transformed with a high copy vector that drives overexpression of the Pho4 transactivator. Cells were incubated in low phosphate media at the non-permissive temperature for the indicated periods of time and analyzed by Northern blot.