GAL4 activates gene expression in mammalian cells.

GAL4, a protein that activates transcription in yeast, is shown to activate the mouse mammary tumor virus promoter in mammalian cells. Activation depends upon a GAL4-binding sequence inserted upstream of the gene. Deletion mutants of GAL4 bearing one or both of the "activating regions" required for activation in yeast also activate transcription in mammalian cells. A derivative of GAL4 that binds to DNA but cannot activate transcription in yeast also fails to activate transcription in mammalian cells. We also show that GAL4 and the glucocorticoid receptor activate the mouse mammary tumor virus promoter synergistically.