Fsy1, the sole hexose-proton transporter characterized in Saccharomyces yeasts, exhibits a variable fructose:H(+) stoichiometry.

In the model yeast Saccharomyces cerevisiae, hexose uptake is mediated exclusively by a family of facilitators (Hxt, hexose transporters). Some other Saccharomyces species (e.g. Saccharomyces bayanus and Saccharomyces pastorianus) possess, in addition, a specific fructose transporter (Fsy1, fructose symporter) that has been previously described to function as a proton symporter. In the present work, we compared growth of a yeast strain in which FSY1 occurs naturally in anaerobic, fructose- and glucose-limited chemostat cultures. Especially at low specific growth rates, fructose-proton symport was shown to have a strong impact on the biomass yield on sugar. We subsequently employed energized hybrid plasma membrane vesicles to confirm previous observations concerning the mode of operation and specificity of Fsy1 mediated transport. Surprisingly, these experiments suggested that the carrier exhibits an unusual fructose:H(+) stoichiometry of 1:2. This energetically expensive mode of operation was also found consistently in vivo, in shake flask and in chemostat cultures, and both when Fsy1 is the sole transporter and when the Hxt carriers are present. However, it is observed only when Fsy1 is operating at higher glycolytic fluxes, a situation that is normally prevented by downregulation of the gene. Taken together, our results suggest the possibility that fructose symport with more than one proton may constitute an energetically unfavorable mode of operation of the Fsy1 transporter that, in growing cultures, is prevented by transcriptional regulation.

L-Arabinose transport and catabolism in yeast.

Two yeasts, Candida arabinofermentans PYCC 5603(T) and Pichia guilliermondii PYCC 3012, which show rapid growth on L-arabinose and very high rates of L-arabinose uptake on screening, were selected for characterization of L-arabinose transport and the first steps of intracellular L-arabinose metabolism. The kinetics of L-arabinose uptake revealed at least two transport systems with distinct substrate affinities, specificities, functional mechanisms and regulatory properties. The L-arabinose catabolic pathway proposed for filamentous fungi also seems to operate in the yeasts studied. The kinetic parameters of the initial L-arabinose-metabolizing enzymes were determined. Reductases were found to be mostly NADPH-dependent, whereas NAD was the preferred cofactor of dehydrogenases. The differences found between the two yeasts agree with the higher efficiency of L-arabinose metabolism in C. arabinofermentans. This is the first full account of the initial steps of L-arabinose catabolism in yeast including the biochemical characterization of a specific L-arabinose transporter.

Differential regulation by glucose and fructose of a gene encoding a specific fructose/H+ symporter in Saccharomyces sensu stricto yeasts.

Saccharomyces cerevisiae transports fructose through a facilitated diffusion system common to other hexoses and mediated by the Hxt proteins. The related species S. pastorianus (carlsbergensis) and S. bayanus produce, in addition, a specific fructose/H(+) symporter. We have previously cloned a gene (FSY1) encoding the active fructose symporter from S. pastorianus PYCC 4457. Expression of Fsy1p in a S. cerevisiae mutant (hxt-null) devoid of the facilitated diffusion system allows growth on fructose but not on glucose. Here we present results concerning the regulation of Fsy1p expression, both in S. pastorianus and S. bayanus, where it occurs naturally, and in suitably engineered S. cerevisiae transformants. To that purpose, we made use of both Northern blot analysis and a Fsy1p-GFP fusion protein. The expression of Fsy1p is strongly regulated by both the carbon source and its concentration in the growth medium. In S. pastorianus, as well as in S. bayanus, very low concentrations of either fructose or glucose induced expression but higher sugar concentrations prevented transcription of the gene. Glucose was considerably more effective than fructose in repressing FSY1 expression. Proper regulation of the gene in S. cerevisiae seems to be exquisitely dependent on sugar transport. Analysis of Fsy1 expression in S. cerevisiae mutants shows that repression is mainly dependent on Mig1p, the final effector of the main glucose repression pathway. Interestingly, Mig1p also seems to mediate repression of FSY1 expression by high maltose concentrations.