Yeast Zinc Cluster Proteins Dal81 and Uga3 Cooperate by Targeting Common Coactivators for Transcriptional Activation of Γ-Aminobutyrate Responsive Genes

Abstract

In Saccharomyces cerevisiae, optimal utilization of various compounds as a nitrogen source is mediated by a complex transcriptional network. The zinc cluster protein Dal81 is a general activator of nitrogen metabolic genes, including those for γ-aminobutyrate (GABA). In contrast, Uga3 (another zinc cluster protein) is an activator restricted to the control of genes involved in utilization of GABA. Uga3 binds to DNA elements found in the promoters of target genes and increases their expression in the presence of GABA. Dal81 appears to act as a coactivator since the DNA-binding activity of this factor is dispensable but its mode of action is not known. In this study, we have mapped a regulatory, as well as an activating, region for Uga3. A LexA-Uga3 chimeric protein activates a lexA reporter in a GABA- and Dal81-dependent manner. Activation by Uga3 requires the SAGA complex as well as Gal11, a component of mediator. ChIP analysis revealed that Uga3 is weakly bound to target promoters. The presence of GABA enhances binding of Uga3 and allows recruitment of Dal81 and Gal11 to target genes. Recruitment of Gal11 is prevented in the absence of Dal81. Importantly, Dal81 by itself is a potent activator when tethered to DNA and its activity depends on SAGA and Gal11 but not Uga3. Overexpression of Uga3 bypasses the requirement for Dal81 but not for SAGA or Gal11. Thus, under artificial conditions, both Dal81 and Uga3 can activate transcription independently of each other. However, under physiological conditions, both factors cooperate by targeting common coactivators.