Gluconeogenic gene regulation in Saccharomyces cerevisiae
The yeast FBP1 and PCK1 genes and the gluconeogenic enzymes that they encode, fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase, are subject to multiple levels of regulation by glucose. It has been reported that transcriptional repression of these genes is exceptionally sensitive to glucose, being triggered by glucose concentrations of less than 0.005% (0.25 mM). It was shown here that in addition at transcriptional repression, the FBP1 and PCK1 and mRNAs are destabilised about 2-fold upon addition of the same low levels of glucose. Low levels of the fermentable sugars fructose or sucrose also stimulated this effect but galactose did not. This destabilisation was lost in a triple hxk1, hxk2, glk1 mutant, but was not triggered by addition of 2-deoxyglucose. The data suggests that sugar phosphorylation and further metabolism of glucose is required to trigger this response. Analysis of metabolic mutants showed that mutations in the upper part of the glycolytic pathway abolish the destabilisation of the FBP1 mRNA. Differences were shown to exist between the regulatory pathways that mediate glucose-stimulated mRNA decay and transcriptional repression. Models which might account for the mechanisms by which rapid decay of the gluconeogenic mRNAs is triggered are discussed. A strategy based on gene fusions with the stable PGK1 mRNA was designed in order to map cis-acting regions which influence PCK1 mRNA stability. A fusion mRNA containing the PCK1 mRNA protein coding region was not destabilised upon addition of low levels of glucose. It was therefore suggested that glucose-stimulated mRNA decay might in some way be dependent upon translation initiation via an interaction with the 5'-leader.