Isolation and preliminary characterization of the GAL4 gene, a positive regulator of transcription in yeast.

The GAL4 locus encodes a positive regulator of the inducible galactose and melibiose genes of yeast. Using the yeast plasmid vector YEp13, we have cloned GAL4 by complementation of a gal4 mutation. Restriction endonuclease mapping of subclone DNA has delimited the region sufficient for complementation to a 3.2-kilobase segment of DNA. The GAL4 mRNA is 2.8 kilobases long, sufficient to encode a protein as large as 105,000 daltons. The concentration of the GAL4 transcript is about 0.1 per cell and is almost identical in galactose-induced and noninduced cells. This result is consistent with a previously proposed model in which the activity of the GAL4 protein and not the transcription of the GAL4 gene is modulated by galactose induction.