Expression of HXT5 is determined by growth rates in
Saccharomyces cerevisiae .
René Verwaal, Hans Paalman, Rick Kapur, Astrid Hogenkamp, Arie
Verkleij, Theo Verrips, Johannes Boonstra
Molecular Cell Biology, Utrecht University, Padualaan 8, Utrecht, 3584
CH, The Netherlands
In Saccharomyces cerevisiae, hexose transporters mediate
transport of glucose across the plasma membrane. A multigene family that
includes HXT1-17, GAL2, SNF3 and RGT2
encodes these transporters. The proteins encoded by HXT1-4 and
HXT6-7, whose expression is regulated by extracellular glucose
concentrations, are the main contributors to glucose transport. One of
the remaining hexose transporters, encoded by HXT5, also has
glucose transport capacity. However, we found that expression of
HXT5 mRNA and Hxt5 protein was not regulated by the extracellular
glucose concentration, but by the growth rate of cells. During batch
growth, expression of HXT5 was induced prior to glucose
depletion, simultaneously with a decrease in the growth rate of cells.
Increasing the temperature or osmolarity, conditions that are known to
decrease growth, induced expression of HXT5. Decreasing the
growth rate to low rates in a fed-batch culture also resulted in
expression of HXT5. Definite proof that showed that indeed the
growth rate determined expression of HXT5 was obtained from N-limited continuous culture experiments, where HXT5 was expressed
only at low growth rates (D<0.1). The promoter of HXT5 contains
putative regulatory elements including one PDS element, two STRE
elements and two HAP2/3/4/5 binding sites, which may be involved in the
regulation of HXT5 expression. Therefore, we are currently
determining whether these putative elements contribute to expression of
HXT5 during low growth rates.
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