Active Snf1 protein kinase inhibits expression of the Saccharomyces cerevisiae HXT1 glucose transporter gene.

Expression of HXT1, a gene encoding a Saccharomyces cerevisiae low-affinity glucose transporter, is regulated by glucose availability, being activated in the presence of glucose and inhibited when the levels of the sugar are scarce. In this study we show that Snf1 protein kinase participates actively in the inhibition of HXT1 expression. Activation of Snf1, either by physiological conditions (growth in low-glucose conditions) or by eliminating any of its negative regulators, such as Hxk2 or Reg1, leads to an inhibition of HXT1 expression. We also show that Std1, another known negative regulator of HXT1 expression, interacts physically with active Snf1 protein kinase. Std1 also interacts physically with Rgt1, a transcription factor involved in HXT1 expression, suggesting that the transcriptional properties of Rgt1 could be modulated either directly or indirectly by Std1 and Snf1 protein kinase. Finally, we show that Rgt1 interacts physically with Ssn6, a major transcriptional repressor, to regulate negatively HXT1 expression when glucose is depleted.