A synergistic increase in potency of a multimerized VP16 transcriptional activation domain.

Transcriptional synergy in eucaryotes provides a means to control both the level and diversity of gene expression. The mechanism by which multiple activators elicit such effects is unknown. To address this problem we considered whether multimerizing an activation domain was equivalent to oligomerizing an activator's binding sites on DNA. Synthetic activators bearing one, two or four VP16 'core' activation domains, fused to the GAL4 DNA binding domain, were co-transfected into Cos-1 cells with CAT gene reporter templates containing one, two or five upstream GAL4 binding sites. Our results demonstrate that all of the activators elicit synergistic effects when comparing the amounts of transcription on multiple sites versus a single site. In contrast, the multimerized activation domains did not stimulate transcription significantly on a template bearing a single site; a synergistic increase in potency was, however, apparent on a template bearing two sites. Introducing the flexible lambda repressor linker region in between the activation domains increased the ability of activators bearing two or four VP16 domains to stimulate transcription from the single-site template. We discuss the mechanistic implications of this study on gene activation and synergy.