S. Ozcan et M. Johnston, 3 DIFFERENT REGULATORY MECHANISMS ENABLE YEAST HEXOSE TRANSPORTER (HXT) GENES TO BE INDUCED BY DIFFERENT LEVELS OF GLUCOSE, Molecular and cellular biology, 15(3), 1995, pp. 1564-1572
The HXT genes (HXT1 to HXT4) of the yeast Saccharomyces cerevisiae enc
ode hexose transporters, We found that transcription of these genes is
induced 10- to 300-fold by glucose. Analysis of glucose induction of
HXT gene expression revealed three types of regulation: (i) induction
by glucose independent of sugar concentration (HXT3); (ii) induction b
y low levels of glucose and repression at high glucose concentrations
(HXT2 and HXT4); and (iii) induction only at high glucose concentratio
ns (HXT1). The lack of expression of all four HXT genes in the absence
of glucose is due to a repression mechanism that requires Rgt1p and S
sn6p. GRR1 seems to encode a positive regulator of HXT expression, sin
ce grr1 mutants are defective in glucose induction of all four HXT gen
es. Mutations in RGT1 suppress the defect in HXT expression caused by
grr1 mutations, leading us to propose that glucose induces HXT express
ion by activating Grr1p, which inhibits the function of the Rgt1p repr
essor. HXT1 expression is also induced by high glucose levels through
another regulatory mechanism: rgt1 mutants still require high levels o
f glucose for maximal induction of HXT1 expression. The lack of induct
ion of HXT2 and HXT4 expression on high levels of glucose is due to gl
ucose repression: these genes become induced at high glucose concentra
tions in glucose repression mutants (hxk2, reg1, ssn6, tup1, or mig1).
Components of the glucose repression pathway (Hxk2p and Reg1p) are al
so required for generation of the high-level glucose induction signal
for expression of the HXT1 gene. Thus, the glucose repression and gluc
ose induction mechanisms share some of the same components and may sha
re the same primary signal generated from glucose.