EVIDENCE THAT COMPLEX-FORMATION BY BAS1P AND BAS2P (PHO2P) UNMASKS THE ACTIVATION FUNCTION OF BAS1P IN AN ADENINE-REPRESSIBLE STEP OF ADE GENE-TRANSCRIPTION

Bas1p and Bas2p (Pho2p) are Myb-related and homeodomain DNA binding pr oteins, respectively, required for transcription of adenine biosynthet ic genes in Saccharomyces cerevisiae. The repression of ADE genes in a denine-replete cells involves down-regulation of the functions of one or both of these activator proteins. A LexA-Bas2p fusion protein was f ound to activate transcription from a lexAop-lacZ reporter independent ly of both BAS1 function and the adenine Levels in the medium. In cont rast, a LexA-Bas1p fusion activated the lexAop reporter in a BAS2-depe ndent and adenine-regulated fashion. The DNA binding activity of Bas2p was not needed for its ability to support activation of the lexAop re porter by LexA-Bas1p, indicating that LexA-Bas1p recruits Bas2p to thi s promoter. The activation functions of both authentic Bas1p and LexA- Bas1p were stimulated under adenine-repressing conditions by overexpre ssion of Bas2p, suggesting that complex formation by these proteins is inhibited in adenine-replete cells. Replacement of Asp-617 with Asn i n Bas1p or LexA-Bas1p allowed either protein to activate transcription under repressing conditions in a manner fully dependent on Bas2p, sug gesting that this mutation reduces the negative effect of adenine on c omplex formation by Bas1p and Bas2p. Deletions of N-terminal and C-ter minal segments from the Bas1p moiety of LexA-Bas1p allowed high-level activation by the truncated proteins independently of Bas2p and adenin e levels in the medium. From these results we propose that complex for mation between Bas1p and Bas2p unmasks a latent activation function in Bas1p as a critical adenine-regulated step in transcription of the AD E genes.