Identification of the structural gene for glucose-6-phosphate dehydrogenase in yeast. Inactivation leads to a nutritional requirement for organic sulfur.

Cloning of the MET19 gene revealed that it encodes the glucose-6-phosphate dehydrogenase from yeast. Sequence analysis showed a high degree of similarity between the yeast and the human enzymes. The cloned gene has allowed the construction of a glucose-6-phosphate dehydrogenase null mutant. The only phenotype of such a strain is an absolute requirement for an organic sulfur source, i.e. methionine, S-adenosylmethionine (AdoMet), cysteine, glutathione or homocysteine. The phenotype of this null mutant raises some new questions about the exact functions of the pentose phosphate pathway which was usually considered as the main cellular source of NADPH. Moreover, results reported here show that an increase of the AdoMet pool represses the transcription of the glucose-6-phosphate dehydrogenase gene. This regulation acts on the glucose-6-phosphate dehydrogenase biosynthesis but does not affect the synthesis of 6-phosphogluconate dehydrogenase. That AdoMet controls a part of the pentose phosphate pathway sheds new light on mechanisms regulating the relative fluxes of carbon utilization through the pentose phosphate pathway and glycolysis.