Characterization of two different types of UDP-glucose/-galactose 4-epimerase involved in galactosylation in fission yeast

Schizosaccharomyces species are currently the only known organisms with two types of genes encoding UDP-glucose/-galactose 4-epimerase, uge1⁺ and gal10⁺. A strain deleted for uge1⁺ exhibited a severe galactosylation defect and a decrease in activity and in UDP-galactose content when grown in glucose-rich medium (2% glucose), indicating that Uge1p is a major UDP-glucose/-galactose 4-epimerase under these growth conditions. In contrast, gal10⁺ was efficiently expressed and involved in galactosylation of cell-surface proteins in low-glucose medium (0.1% glucose and 2% glycerol), but not in galactose-containing medium. In a uge1Δgal10Δ strain, the galactosylation defect was suppressed and UDP-galactose content restored to wild-type levels in galactose-containing medium. Disruption of gal7⁺, encoding galactose-1-phosphate uridylyltransferase, in the uge1Δ gal10Δ strain reversed suppression of the galactosylation defect and reduced levels of UDP-galactose, indicating that galactose is transported from the medium to the cytosol and is converted into UDP-galactose via galactose 1-phosphate by Gal7p in Sch. pombe.