The
yeast Saccharomyces cerevisiae responds to galactose as the sole
source of carbon by activating the genes encoding the enzymes of the
Leloir pathway. This process depends on a transcriptional activator,
Gal4p. In the absence of galactose, the activity of Gal4p is inhibited
by a repressor, Gal80p. The switch from repressed to active
transcription involves another protein, Gal3p. We have studied the
switch between repressed and activated transcription using purified
proteins to elucidate the mechanism of galactose induction. We find that
Gal4p-mediated transcriptional activity is specifically inhibited by
Gal80p using an in vitro transcription assay system. This
inhibition is alleviated by Gal3p in the presence of galactose and
nucleotides. A constitutive mutant of Gal3p shows galactose-independent
relief of Gal80p repression, while uninducible mutants of Gal80p are not
susceptible to the effects of Gal3p. Gal3p is highly homologous to the
yeast galactokinase, Gal1p, although it does not possess galactokinase
function. We find that Gal1p will also relieve Gal80p repression of
Gal4p, although at considerably reduced efficiency compared to Gal3p.
Finally, we show that Gal3p will form a complex with DNA-bound
Gal4p/Gal80p in a galactose- and ATP-dependant manner, suggesting that
galactose induction of Gal4p occurs through the interaction between
Gal3p and Gal80p. Gal80p does not dissociate from Gal4p, and is
therefore likely to undergo a conformational change to allow Gal4p to
activate transcription.
Return to YGM 1998 Abstract Index