Identification and characterization of the activation domain of Ifh1, an activator of model TATA-less genes

In yeast, TATA box-binding protein TBP can be delivered to protein-coding genes by direct interactions with two different coactivators: TFIID, which delivers TBP preferentially to TATA-less promoters, and SAGA, which strongly favors TATA box-containing promoters. Transcriptional activators of SAGA-dependant genes are characterized by prototypic acidic activation domains (ADs) that efficiently recruit SAGA, but not TFIID, to UAS elements even in the absence of a core promoter. In contrast to the well-studied acidic activation domains, little is known about the activation domains of activators of TFIID-dependent genes, even though these genes constitute more than 80% of eukaryotic protein-coding genes. The paradigm for TATA-less genes are the ribosomal protein genes (RPGs). Here we have identified the AD of the RPG activator Ifh1p and demonstrate that a minimal Ifh1 AD represents a new class of AD that significantly differs from acidic ADs in amino acid signature, relative coactivator affinities, and core promoter selectivity.