The FBPl and PCKl genes encode the gluconeogenic enzymes fructose-l,6-bisphosphatase and phosphoenoZpyruv9te carboxykinase, respectively. In the yeast, Saccharomyces cerevisiae, the corresponding mRNAs are present at low levels during growth on glucose, but are present at elevated levels during growth on gluconeogenic carbon sources. We demonstrate that the levels of the FBPl and PCKl mRNAs are acutely sensitive to the addition of glucose to the medium and that the levels of these mRNAs decrease rapidly when glucose is added to the medium at a concentration of only 0.005%. At this concentration, glucose blocks FBPl and PCKl transcription, but has no effect on iso-1 cytochrome c (CYCI) mRNA levels. Glucose also increases the rate of degradation of the PCKl mRNA approximately twofold, but only has a slight effect upon FBPl mRNA turnover. We show that the levels of the FBPl and PCKl mRNAs are also sensitive to other environmental factors. The levels of these mRNAs decrease 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°C to 36°C). The latter response appears to be mediated by accelerated mRNA decay.Glycolysis and gluconeogenesis constitute two antagonistic pathways for the metabolism of different carbon sources. Depending on the resources available to the yeast, one pathway or the other is thought to be operational since their simultaneous function would probably cause futile cycles at the level of the antagonistic enzyme pairs phosphofructokinase/fructose-1,6-bisphosphatase and pyruvate kinaselphosphoenoZpyruvate carboxykinase. Tight regulation of these enzymes might therefore be expected, and in fact the enzymes are subject to multiple mechanisms of control, including allosteric regulation, protein inactivation, and changes in enzyme levels [l]. Glucose increases the levels of phosphofructokinase and pyruvate kinase mRNAs due to changes in the transcription rates of the corresponding genes [2]. Also, glucose-grown yeast has barely detectable levels of the mRNAs corresponding to the gluconeogenic genes FBPl and PCKl ,which encode fructose-l,6-bisphosphatase and phosphoenoZpyruvate carboxykinase, respectively [3 -51. Although it is usually assumed that these low levels are due to repression of transcription caused by glucose, glucose could also affect the stability of the mRNAs. To address this question, we measured the half-lives of the gluconeogenic mRNAs in yeast growing on non-fermentable carbon sources and in yeast shifted to a medium with glucose. In this study, we show that the turnover of the FBPl and PCKl mRNAs is accelerated slightly in the presence of glucose, but that the major response is at the transcriptional level.In the course of the experiments, we observed that the addition of hot fresh medium to yeast strains growing on a non-fermentable carbon source caused the rapid disappearance of the FBPl and PCKl mRNAs. Therefore, we studied the factors which could influence the levels of these gluconeogenic mRNAs in yeast. These...

By deletion analysis of the fusion genes FBPI-f&Z and PCKI-incZ we have idcniificd a number of strong regulatory regions in the gcncs FBPI and PCKI which encode fructose-l,6-bisphosphatase and phosphoenolpyruvate carboxykinasc. Lack of expression of,&galaciosidase in fusions lacking sequences from the coding regions suggests the existence of downstream activating elements. Both promoters have scvcral UAS and URS regions as well as sites implicated in catabolitc repression. We have round in both genes consensus sequences ror the binding of the same regulatory proteins, such as yAPI, MIGI or the complex HAPZ/HAP3/HAP4. Neither deletion nor overexpression of the AJIGf gene affected the regulated expression of lhe FBPf or PCKf genes.

We have identified in the promoter of the yeast FBPl gene two sites able to bind nuclear proteins. These sites have a nucleotide sequence strongly similar to that of sites which bind the regulatory protein MIGl in the promoters of GAL4 and SUCZ. Deletions performed in the FBPI promoter showed that one of the sites contributes to catabolite repression of this gene. In the same promoter, another region was identified with a strong effect on the catabolite repression of FBPI. In this region a sequence similar to the consensus for the binding site of the MIGl protein was also present.