ACTIVATION DOMAIN-MEDIATED ENHANCEMENT OF ACTIVATOR BINDING TO CHROMATIN IN MAMMALIAN-CELLS

DNA binding by transcriptional activators is typically an obligatory s tep in the activation of gene expression, Activator binding and subseq uent steps in transcription are repressed by genomic chromatin. Studie s in vitro have suggested that overcoming this repression is an import ant function of some activation domains. Here we provide quantitative in vivo evidence that the activation domain of GAL4-VP16 can increase the affinity of GAL4 for its binding site on genomic DNA in mammalian cells. Moreover, the VP16 activation domain has a much greater stimula tory effect on expression from a genomic reporter gene than on a trans iently transfected reporter gene, where factor binding is more permiss ive, We found that not all activation domains showed a greater activat ion potential in a genomic context, suggesting that only some activati on domains can function in vivo to alleviate the repressive effects of chromatin, These data demonstrate the importance of activation domain s in relieving chromatin-mediated repression in vivo and suggest that one way they function is to increase binding of the activator itself.