A Ga14-sigma(54) hybrid protein that functions as a potent activator of RNA polymerase II transcription in yeast

The bacterial sigma (54) protein associates with core RNA polymerase to for m a holoenzyme complex that renders cognate promoters enhancer-dependent. A lthough unusual in bacteria, enhancer-dependent transcription is the paradi gm in eukaryotes. Here we report that a fragment of Escherichia coli sigma (54) encompassing amino acid residues 29-177 functions as a potent transcri ptional activator in yeast when fused to a Gal4 DNA binding domain. Activat ion by Gal4-sigma (54) is TATA-dependent and requires the SAGA coactivator complex, suggesting that Gal4-sigma (54) functions by a normal mechanism of transcriptional activation. Surprisingly, deletion of the AHC1 gene, which encodes a polypeptide unique to the ADA coactivator complex, stimulates Ga l4-sigma (54)-mediated activation and enhances the toxicity of Gal4-sigma ( 54). Accordingly, the SAGA and ADA complexes, both of which include Gcn5 as their histone acetyltransferase subunit, exert opposite effects on transcr iptional activation by Gal4-sigma (54). Gal4-sigma (54) activation and toxi city are also dependent upon specific sigma (54) residues that are required for activator-responsive promoter melting by sigma (54) in bacteria, imply ing that activation is a consequence of sigma (54)-specific features rather than a structurally fortuitous polypeptide fragment. As such, Gal4-sigma ( 54) represents a novel tool with the potential to provide insight into the mechanism by which natural activators function in eukaryotic cells.