Facilitated diffusion of 6-deoxy-d-glucose by the oxidative yeast, Kluyveromyces lactis

Glucose transport in the yeast Kluyveromyces lactis. II. Transcriptional regulation of the glucose transporter gene RAG1

The RAG1 gene encodes a membrane protein involved in the low-affinity glucose/fructose transport system of the yeast Kluyveromyces lactis. Analysis of steady-state mRNA levels analysis and quantitation of expression by beta-galactosidase from RAG1-lacZ fusions assays revealed that the RAG1 gene was poorly expressed in cells grown under gluconeogenesis conditions, but was induced more than ten-fold when they were grown on various sugars. These sugars included glucose, fructose, mannose, sucrose, raffinose, as well as galactose. Nucleotide sequence and deletion analysis of the 5' flanking region of the RAG1 gene showed that an essential cis-acting element required for induced transcription of the RAG1 gene resided between -615 and -750 from the coding sequence. This region contained a 22 bp purine stretch, and a pair of 11 bp direct repeat sequences. The 11 bp repeats harbor a CCAAT motif, a consensus sequence for binding of the yeast and mammalian HAP2/3/4-type protein complex. The transcription of the RAG1 gene was dramatically affected by three unlinked mutations, rag4, rag5 and rag8. We discuss the possible roles of RAG4, RAG5 and RAG8 gene products in the expression of the RAG1 gene, as well as the importance of the inducible RAG1 gene in the fermentative growth of K. lactis.

Asymmetry of glucose transport in the yeast, Kluyveromyces lactis

Uptake and efflux of 6-deoxy-D-[3H]glucose and of 2-deoxy-D-[14C]glucose by the yeast Kluyveromyces lactis was studied. The tritiated, nonphosphorylatable hexose analogue leaves the cell in the absence and presence of intracellular 2-deoxy-D-glucose 6-phosphate. In energy-rich cells containing pools of hexose 6-phosphate, 2-deoxy-D-glucose is trapped in the cells, for it neither effluxes into glucose-free medium nor exchanges with external, free sugar. In starved, poisoned cells containing negligible amounts of 2-deoxy-D-glucose 6-phosphate, 2-deoxy-D-glucose does leave the cells upon transfer to glucose-free medium. An involvement of analogue structure and availability of metabolites of energy-rich cells in hexose retention is suggested. An internal pool of 6-deoxy-D-glucose does not affect the rate of uptake of 6-deoxy-D-[3H]glucose, nor does internal 2-deoxy-D-[14C]glucose 6-phosphate influence that rate. Hence, transport of glucose by this yeast is probably not regulated by internal pools of glucose 6-phosphate.

Glucose transport in nongrowing yeast

The inducible, nonenergy-requiring glucose transport system of the yeast Kluyveromyces lactis is inactivated upon starving cells of glucose by (1) transferring logarithmic phase glucose-grown cells to synthetic medium containing a nonglycolytic carbon source, and (2) upon transition of logarithmic phase glucose-grown cells to stationary phase. The steady-state accumulation of nonmetabolizeable 6-deoxyglucose and the apparent Km of transport of 6-deoxyglucose is the same in stationary phase cells and in logarithmic phase cells. The rate of transport is lower in the nongrowing cells. Restoration of activity requires energy and protein synthesis as well as inducer.