POLY(DA-DT), A UBIQUITOUS PROMOTER ELEMENT THAT STIMULATES TRANSCRIPTION VIA ITS INTRINSIC DNA-STRUCTURE

Many yeast promoters contain homopolymeric dA:dT sequences that affect nucleosome formation in vitro and are required for wild-type levels o f transcription in vivo. Here, we show that poly(dA:dT) is a novel pro moter element whose function depends on its intrinsic structure, not i ts interaction with sequence-specific, DNA-binding proteins. First, po ly(dA:dT) stimulates Gcn4-activated transcription in a manner that is length dependent and inversely related to intracellular Gcn4 levels. S econd, Datin, the only known poly(dii:dT)-binding protein, behaves as a repressor through poly(dA:dT) sequences. Third, poly(dG:dC), a struc turally dissimilar homopolymer that also affects nucleosomes, has tran scriptional properties virtually identical to those of poly(dA:dT), Th ree probes of chromatin structure including HinfI endonuclease cleavag e in vivo indicate that poly(dA:dT) increases accessibility of the Gcn 4 binding site and adjacent sequences in physiological chromatin. Thes e observations suggest that, by virtue of its intrinsic structure, pol y(dA:dT) locally affects nucleosomes and increases the accessibility o f transcription factors bound to nearby sequences.