Signaling through regulated transcription factor interaction: mapping of aregulatory interaction domain in the Myb-related Bas1p

Gene activation in eukaryotes is inherently combinatorial depending on coop eration between different transcription factors. An example where this coop eration seems to be directly exploited for regulation is the Bas1p/Bas2p co uple in yeast. Bas1p is a Myb-related transcription factor that acts togeth er with the homeodomain-related Bas2p (Pho2p) to regulate purine and histid ine biosynthesis genes in response to extracellular purine limitation. We s how that fusion of the two factors abolished adenine repression, suggesting that what is regulated by adenine is the Bas1p-Bas2p interaction. Analysis of Bas1p deletions revealed a critical domain (Bas1p interaction and regul atory domain, BIRD) acting in two-hybrid assays as an adenine-dependent Bas 1p-Bas2p interaction domain. BIRD had a dual function, as an internal repre ssor of a centrally located Bas1p transactivation domain on the ADE1 promot er and as a Bas2p-dependent activator on the HIS4 promoter. This promoter-d ependent behavior reflected a differential binding to the two promoters in vivo. On ADE1 Bas1p bound the promoter efficiently by itself, but required adenine limitation and Bas2p interaction through BIRD for derepression. On HIS4 efficient promoter binding and derepression required both factors and adenine limitation. We propose a promoter-dependent model for adenine regul ation in yeast based on controlled Bas1p-Bas2p interactions through BIRD an d exploited differentially by the two promoters.