Microbiology 314 Van Zyl

GAL regulation:

GAL/MEL regulon of yeast:

- The galactose/melibiose (GAL/MEL) regulon of S. cerevisiae is one of the best characterised
eukaryotic systems of transcriptional regulation
- Structural genes of this regulon, GAL genes are regulated at transcriptional level in a
carbon-dependent manner
- Yeast growing in a non-fermentable carbon source, such as ethanol or glycerol, don’t
express GAL genes
- Addition of galactose causes the rapid induction of GAL genes
- However, expression of GAL genes is rapidly shut off upon addition of glucose – known as
glucose repression
- Galactose is transported across the membrane by the permease encoded by GAL2
- Intracellular galactose is converted
to glucose-1-phosphate by the
sequential action of galatokinase,
galactose-1-phosphate uridyl
transferase and uridine
diphosphoglucose epimerase – these
enzymes are encoded by GAL1, Gal7
and GAL10 respectively
- Phosphoglucomutase encoded by
GAL5, converts G-1-P  G-6-P which
then enters the glycolytic pathway
- Melibiose is hydrolysed by α-
galactosidase (encoded by MEL1),
to glucose and galactose and the
released galactose enters the
glycolytic pathway
- GAL2, GAL1, GAL7, GAL10 and MEL1 are the
GAL structural genes
- Although the three genes (GAL7,10 and 1) lie
adjacent to one another, each forms a separate
transcription unit i.e. monocistronic units
- Gal4 acts as an activator and promotes
transcription of GAL genes
- Gal80 (co-repressor) interacts with Gal4 and prohibits binding (thus activation) of GAL
genes




o Gal4 and Gal80 are made in the cytplasm
o GAL genes are made in the nucleus and have
upstream acitivating sequences that are recognised by
Gal4




© Lisa Stein