TRANSCRIPTIONAL ACTIVATION BY YEAST PDR1P IS INHIBITED BY ITS ASSOCIATION WITH NGG1P ADA3P/

NGG1p/ADA3p forms a coactivator/repressor complex (ADA complex) in ass ociation with at least two other yeast proteins, ADA2p and GCN5p, that is involved in regulating transcriptional activator proteins includin g GAL4p and GCN4p. Using a two-hybrid analysis, we found that the carb oxyl-terminal transcriptional activation domain of PDR1p, the primary regulatory protein involved in yeast pleiotropic drug resistance, inte racts with the amino-terminal 373 amino acids of NGG1p (NGG1p(1-373)), This interaction was confirmed by coimmunoprecipitation of epitope-ta gged derivatives of NGG1p and PDR1p from crude extracts. An overlappin g region of the related transcriptional activator PDR3p was also found to interact with NGG1p. Amino acids 274-307 of NGG1p were required fo r interaction with PDR1B, This same region is required for inhibition of transcriptional activation by GAL-lp. The association between NGG1p (1-373) and PDR1p may be indirect, possibly mediated by the ADA comple x since the two-hybrid interaction required the presence of full-lengt h NGG1. A partial requirement for ADA2 was also found. This suggests t hat an additional component of the ADA complex, regulated by ADA2p, ma y mediate the interaction. Transcriptional activation by a GAL4p DNA b inding domain fusion of PDR1p was enhanced in ngg1 and ada2 disruption strains, Similar to its action on GAL4p, the ADA complex acts to inhi bit the activation domain of PDR1p.