Analysis of Saccharomyces cerevisiae hexose carrier expression during wine fermentation: both low- and high-affinity Hxt transporters are expressed

The transport of glucose and fructose into yeast cells is a critical step in the utilization of sugars during wine fermentation. Hexose uptake can be carried out by various Hxt carriers, each possessing distinct regulatory and transport-kinetic properties capable of influencing yeast fermentation capacity. We investigated the expression pattern of the hexose transporters Hxt1 to 7 at the promoter and protein levels in Saccharomyces cerevisiae during wine fermentation. The Hxt1p carrier was expressed only at the beginning of fermentation, and had no role during stationary phase. The Hxt3p carrier was the only one to be expressed throughout fermentation, displaying maximal expression at growth arrest and slowly decreasing in abundance over the course of the stationary phase. The high-affinity carriers Hxt6p and Hxt7p displayed similar expression profiles, with expression induced at entry into stationary phase and persisting throughout the phase. The expression of these two carriers occurred despite the presence of high amounts of hexoses, and the proteins were stably expressed when the cells were starved for nitrogen. The Hxt2p transporter was only transiently expressed during lag phase, which suggests a role for the protein in growth initiation. Characterization of glucose transport kinetics indicated the presence of a shift in the low-affinity component that is consistent with a predominant expression of Hxt1p during growth phase and of Hxt3p during stationary phase. In addition, a high-affinity uptake component consistent with functional expression of Hxt6p/Hxt7p was identified during stationary phase.