Experiment demonstrating the separate DNA-binding and regulatory domains of transcription activators. (a) A GAL1-lacZ fusion gene is inserted in a plasmid downstream from a LexA-binding site (top), or downstream from a DNA segment lacking the LexA-binding site (bottom). In both cases, cells are transformed with a plasmid encoding a fusion protein containing the bacterial LexA DNA-binding domain fused to the transcription-activation region of yeast Gal4p. (b) Expression of the GAL1-lacZ fusion gene is measured as described in the text. Expression of β-galactosidase is induced in cells containing plasmids with the LexA-binding site.