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Galactose is transported into the cell by the product of GAL2 (permease)
Galactose
GAL2
where it is converted to an inducer (I) by the product of GAL3. The Gal3
GAL3 The Gal3
and Gal5 genes are not regulated by galactose but GAL3 acts as an
Gal5
GAL3
inhibitor of Gal80.
Gal80
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 36 Pathway of Galactose Utilization: 6 Key Genes GAL2
permease
galactose
external galactose
internal α-galactosidase
MEL1
-Gal-Glumelibiose GAL1
kinase GAL7
transferase Gal-1-P UDP-Glu GAL5
mutase Glu-1-P Glu-6-P UDP-Gal epimerase
GAL10
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez glycolysis
37 Model for Gal Gene Regulations The model for GAL gene regulation in yeast is based on the following
The
GAL
observations:
(1) GAL genes are expressed in abundance when yeast cultures are
GAL
shifted from galactose-free to galactose-containing media.
(2) Unlinked genes (GAL4 and GAL80) affect the expression of these
GAL80 affect
genes. GAL4 is expressed constitutively and activates transcription of
the GAL1, 2, 7, 10 and MEL1 genes by binding the UAS enhancers. In
GAL1
10
MEL1
this position, the carboxy-terminal end of GAL4 interacts with a TATA box
binding factor (TBF).
binding
).
(3) The TBF interacts with RNAPII to stimulate transcription of the
adjacent gene. GAL80 interacts with the C-terminus of GAL4 to prevent
activation of the TBF/RNAPII complex.
(4) When yeast cells are grown in the presence of glucose (or a
combination of glucose and galactose), the catabolite repressor protein
(CRP) iinteracts with the DNA binding domain of GAL4 to prevent its
nteracts
binding to the UAS.
binding
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 38 Mutation Analysis Mutational analysis has shown that the Gal genes (Gal1, 2, 7,
Mutational
Gal
Gal1,
10 and Mel1) are induced by galactose and not expressed
10
are induced
constitutively.
constitutively
Regulation is accomplished through the action of two additional
Regulation
regulatory proteins, GAL4 and GAL80. Mutations in GAL4
GAL4
GAL80 Mutations
(Gal4-) produce an uninducible phenotype. A Gal80- mutation
produce
Gal80
produces a constitutive phenotype. As shown in the following
figure, the GAL4 protein has a stimulatory affect (shaded
arrows) on Gal gene transcription.
Gal
GAL80 blocks (T-bar) the affect of GAL4. GAL80 is an
GAL80
antagonist of GAL4.
antagonist
The action of GAL80 is blocked when bound by the galactose
The
sensor protein,GAL3 (making GAL3 an antagonist of GAL80).
sensor
GAL3
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013
BIS101001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 39 Components of the Gal Regulatory Circuit II GAL10 GAL7 GAL1 GAL2...
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