THE LEVELS OF YEAST GLUCONEOGENIC MESSENGER-RNAS RESPOND TO ENVIRONMENTAL-FACTORS

The FBP1 and PCK1 genes encode the gluconeogenic enzymes fructose-1,6- bisphosphatase and phosphoenolpyruvate carboxykinase, respectively. In the yeast, Saccharomyces cerevisiae, the corresponding mRNAs are pres ent at low levels during growth on glucose, but are present at elevate d levels during growth on gluconeogenic carbon sources. We demonstrate that the levels of the FBP1 and PCK1 mRNAs are acutely sensitive to t he addition of glucose to the medium and that the levels of these mRNA s decrease rapidly when glucose is added to the medium at a concentrat ion of only 0.005%. At this concentration, glucose blocks FBP1 and PCK 1 transcription, but has no effect on iso-l cytochrome c (CYC1) mRNA l evels. Glucose also increases the rate of degradation of the PCK1 mRNA approximately twofold, but only has a slight effect upon FBP1 mRNA tu rnover. We show that the levels of the FBP1 and PCK1 mRNAs are also se nsitive to other environmental factors. The levels of these mRNAs decr ease transiently in response to a decrease of the pH from pH 7.5 to pH 6.5 in the medium, or to a mild temperature shock (from 24 degrees C to 36 degrees C). The latter response appears to be mediated by accele rated mRNA decay.