Mutations that increase acidity enhance the transcriptional activity of the glutamine-rich activation domain in stage-specific activator protein

Sea urchin stage-specific activator protein (SSAP) activates transcription of the late H1 gene at the mid-blastula stage of development. Its C-terminal 202 amino acids form a potent glycine/glutamine rich activation domain (GQ domain) that can transactivate reporter genes to levels 5-fold higher than VP16 in several mammalian cell lines. We observed that, unlike other glutamine-rich activation domains, the GQ domain activates transcription to moderate levels in yeast. We utilized this activity to screen in yeast for intragenic mutations that enhance or inhibit the transcriptional activity of the GQ domain. We identified 37 loss of function and 23 gain of function mutants. Most gain of function mutations increased the acidity of the domain. The most frequently isolated mutations conferred enhanced transcriptional activity when assayed in mammalian cells. These mutations also enhance the ability of SSAP to up-regulate the late H1 promoter in sea urchin embryos. We conclude that the GQ domain fundamentally differs from other glutamine-rich activators and may share some properties of acidic activators. The ability of acidity to enhance SSAP-mediated transcription may reflect a mechanism by which phosphorylation of SSAP activates late HI gene transcription during embryogenesis.