UDP-galactose 4-epimerase Research Articles

Characterization of UDP-glucose 4-epimerase from Pyrococcus horikoshii: Regeneration of UDP to produce UDP-galactose using two-enzyme system with trehalose

A gene encoding a putative UDP-glucose 4-epimerase (pGALE) in Pyrococcus horikoshii was cloned and expressed in Escherichia coli. The purified enzyme could reversibly catalyze both the synthesis of UDP-Gal and UDP-Glc but preferred the binding of UDP-Gal by approximately 10-fold. The optimum pH and temperature were 6.5 and 65°C. The enzyme acted effectively without the addition of nicotinamide adenine dinucleotide (NAD+), possibly due to the presence of tightly bound NAD+. In particular, pGALE could be coupled with trehalose synthase (TreT) from P. horikoshii to regenerate UDP-Gal from UDP. The possible byproduct of glycosyltransferase, UDP, was capable of being converted to UDP-Glc with trehalose by TreT, and UDP-Glc was simultaneously converted to UDP-Gal by pGALE. Conclusively, the results suggest that pGALE and TreT with trehalose is an effective one-pot two-enzyme system for the regeneration of UDP-Gal, a high-cost substrate of galactosyltransferase, to complete a sugar nucleotide cycle.

To the Happy Few

I became a microbiologist in post-WWII Japan, working with Toshio Fukasawa on galactose metabolism in Salmonella. We characterized mutants defective in UDP-galactose 4-epimerase, which produced a defective lipopolysaccharide, and this opened doors for me to study lipopolysaccharide biosynthesis, which I pursued in the United States, in the laboratory of Herman Kalckar. After I moved to Berkeley, California, in 1969, I became interested in the function of bacterial outer membranes, a line of work that led to the discovery and characterization of porins as well as the studies on the mycobacterial cell wall. In the early 1990s, it became clear that the outer membrane permeability barrier and the activity of periplasmic β-lactamase are not enough to explain the resistance of some strains to β-lactam antibiotics, and the search for the missing factor led to the discovery of RND family multidrug efflux pumps, subjects that continue to fascinate me to this day.

UDP-xylose and UDP-galactose synthesis in Trichomonas vaginalis

The presence of xylose and galactose residues in the structure of trichomonad lipoglycans was indicated by previous studies and the modification of any glycoconjugate with either monosaccharide requires the respective presence of the nucleotide sugars, UDP-xylose and UDP-galactose. Biosynthesis of UDP-xylose de novo is mediated by UDP-xylose synthase (UXS; UDP-glucuronic acid decarboxylase), which converts UDP-glucuronic acid to UDP-xylose, whereas UDP-galactose can be generated from UDP-glucose by UDP-galactose epimerases (GalE). Trichomonas vaginalis cDNAs, encoding proteins with homology to these enzymes from other eukaryotes, were isolated. The recombinant T. vaginalis UDP-xylose synthase and UDP-galactose epimerase were expressed in Escherichia coli and tested via high pressure liquid chromatography to demonstrate their enzymatic activities. Thereby, in this first report on enzymes involved in glycoconjugate biosynthesis in this organism, we demonstrate the existence of xylose and galactose synthesising pathways in T. vaginalis.

A Rice Plastidial Nucleotide Sugar Epimerase Is Involved in Galactolipid Biosynthesis and Improves Photosynthetic Efficiency

Photosynthesis is the final determinator for crop yield. To gain insight into genes controlling photosynthetic capacity, we selected from our large T-DNA mutant population a rice stunted growth mutant with decreased carbon assimilate and yield production named photoassimilate defective1 (phd1). Molecular and biochemical analyses revealed that PHD1 encodes a novel chloroplast-localized UDP-glucose epimerase (UGE), which is conserved in the plant kingdom. The chloroplast localization of PHD1 was confirmed by immunoblots, immunocytochemistry, and UGE activity in isolated chloroplasts, which was approximately 50% lower in the phd1-1 mutant than in the wild type. In addition, the amounts of UDP-glucose and UDP-galactose substrates in chloroplasts were significantly higher and lower, respectively, indicating that PHD1 was responsible for a major part of UGE activity in plastids. The relative amount of monogalactosyldiacylglycerol (MGDG), a major chloroplast membrane galactolipid, was decreased in the mutant, while the digalactosyldiacylglycerol (DGDG) amount was not significantly altered, suggesting that PHD1 participates mainly in UDP-galactose supply for MGDG biosynthesis in chloroplasts. The phd1 mutant showed decreased chlorophyll content, photosynthetic activity, and altered chloroplast ultrastructure, suggesting that a correct amount of galactoglycerolipids and the ratio of glycolipids versus phospholipids are necessary for proper chloroplast function. Downregulated expression of starch biosynthesis genes and upregulated expression of sucrose cleavage genes might be a result of reduced photosynthetic activity and account for the decreased starch and sucrose levels seen in phd1 leaves. PHD1 overexpression increased photosynthetic efficiency, biomass, and grain production, suggesting that PHD1 plays an important role in supplying sufficient galactolipids to thylakoid membranes for proper chloroplast biogenesis and photosynthetic activity. These findings will be useful for improving crop yields and for bioenergy crop engineering.

Identification of alternative splice variants in Aspergillus flavus through comparison of multiple tandem MS search algorithms

BackgroundDatabase searching is the most frequently used approach for automated peptide assignment and protein inference of tandem mass spectra. The results, however, depend on the sequences in target databases and on search algorithms. Recently by using an alternative splicing database, we identified more proteins than with the annotated proteins in Aspergillus flavus. In this study, we aimed at finding a greater number of eligible splice variants based on newly available transcript sequences and the latest genome annotation. The improved database was then used to compare four search algorithms: Mascot, OMSSA, X! Tandem, and InsPecT.ResultsThe updated alternative splicing database predicted 15833 putative protein variants, 61% more than the previous results. There was transcript evidence for 50% of the updated genes compared to the previous 35% coverage. Database searches were conducted using the same set of spectral data, search parameters, and protein database but with different algorithms. The false discovery rates of the peptide-spectrum matches were estimated < 2%. The numbers of the total identified proteins varied from 765 to 867 between algorithms. Whereas 42% (1651/3891) of peptide assignments were unanimous, the comparison showed that 51% (568/1114) of the RefSeq proteins and 15% (11/72) of the putative splice variants were inferred by all algorithms. 12 plausible isoforms were discovered by focusing on the consensus peptides which were detected by at least three different algorithms. The analysis found different conserved domains in two putative isoforms of UDP-galactose 4-epimerase.ConclusionsWe were able to detect dozens of new peptides using the improved alternative splicing database with the recently updated annotation of the A. flavus genome. Unlike the identifications of the peptides and the RefSeq proteins, large variations existed between the putative splice variants identified by different algorithms. 12 candidates of putative isoforms were reported based on the consensus peptide-spectrum matches. This suggests that applications of multiple search engines effectively reduced the possible false positive results and validated the protein identifications from tandem mass spectra using an alternative splicing database.

Enhanced UDP-glucose and UDP-galactose by homologous overexpression of UDP-glucose pyrophosphorylase in Lactobacillus casei

UDP-sugars are widely used as substrates in the synthesis of oligosaccharides catalyzed by glycosyltransferases. In the present work a metabolic engineering strategy aimed to direct the carbon flux towards UDP-glucose and UDP-galactose biosynthesis was successfully applied in Lactobacillus casei. The galU gene coding for UDP-glucose pyrophosphorylase (GalU) enzyme in L. casei BL23 was cloned under control of the inducible nisA promoter and it was shown to be functional by homologous overexpression. Notably, about an 80-fold increase in GalU activity resulted in approximately a 9-fold increase of UDP-glucose and a 4-fold increase of UDP-galactose. This suggested that the endogenous UDP-galactose 4-epimerase (GalE) activity, which inter-converts both UDP-sugars, is not sufficient to maintain the UDP-glucose/UDP-galactose ratio. The L. casei galE gene coding for GalE was cloned downstream of galU and the resulting plasmid was transformed in L. casei. The new recombinant strain showed about a 4-fold increase of GalE activity, however this increment did not affect that ratio, suggesting that GalE has higher affinity for UDP-galactose than for UDP-glucose. The L. casei strains constructed here that accumulate high intracellular levels of UDP-sugars would be adequate hosts for the production of oligosaccharides.

Crystal structure of UDP-galactose 4-epimerase from the hyperthermophilic archaeon Pyrobaculum calidifontis

The crystal structure of a highly thermostable UDP-galactose 4-epimerase (GalE) from the hyperthermophilic archaeon Pyrobaculum calidifontis was determined at a resolution of 1.8Å. The asymmetric unit contained one subunit, and the functional dimer was generated by a crystallographic two-fold axis. Each monomer consisted of a Rossmann-fold domain with NAD bound and a carboxyl terminal domain. The overall structure of P. calidifontis GalE showed significant similarity to the structures of the GalEs from Escherichia coli, human and Trypanosoma brucei. However, the sizes of several surface loops were markedly smaller in P. calidifontis GalE than the corresponding loops in the other enzymes. Structural comparison revealed that the presence of an extensive hydrophobic interaction at the subunit interface is likely the main factor contributing to the hyperthermostability of the P. calidifontis enzyme. Within the NAD-binding site of P. calidifontis GalE, a loop (NAD-binding loop) tightly holds the adenine ribose moiety of NAD. Moreover, a deletion mutant lacking this loop bound NAD in a loose, reversible manner. Thus the presence of the NAD-binding loop in GalE is largely responsible for preventing the release of the cofactor from the holoenzyme.

High level of soluble expression and purification of catalytically active native UDP-galactose 4-epimerase of &amp;lt;i&amp;gt;Aeromonas hydrophila&amp;lt;/i&amp;gt; in &amp;lt;i&amp;gt;E. coli&amp;lt;/i&amp;gt;

The ubiquitous Aeromonas hydrophila is responsible for several pathological conditions in fish and human. Like most gram negative bacteria, its virulence relies on outer membrane lipopolysachharide (LPS). The Leloir pathway enzyme UDP-galactose 4-epimerase (GalE), plays an important role in the LPS biosynthesis, and therefore is a potential drug target. We have earlier carried out extensive biochemical and biophysical studies with histidine-tagged recombinant GalE. However, for effective drug design it is desirable to understand the structure-function relation of a protein in its native form without any additional sequences or tags. In the present study, we report the high level expression, purification and characterization of recombinant GalE (rGalE) of Aeromonas hydrophila in its native form in E coli. The rGalE expressed as a soluble protein was purified to near homogeneity. From 1 L of shake flask culture ~15 mg of purified rGalE was obtained. The purified protein was biologically active with Km and Kcat values of 0.7 mM and 28.8 s–1, respectively. The enzyme exhibited a temperature and pH optima of 37。C and 7 - 9, respectively. Thus, the present study employed for soluble expression and purification of functionally active rGalE without any tag bypasses the need for cumbersome strategies associated with removal of tag from purified protein.

The N-terminus of GalE induces tmRNA activity in Escherichia coli.

BackgroundThe tmRNA quality control system recognizes stalled translation complexes and facilitates ribosome recycling in a process termed ‘ribosome rescue’. During ribosome rescue, nascent chains are tagged with the tmRNA-encoded SsrA peptide, which targets tagged proteins for degradation. In Escherichia coli, tmRNA rescues ribosomes arrested on truncated messages, as well as ribosomes that are paused during elongation and termination.Methodology/Principal FindingsHere, we describe a new translational pausing determinant that leads to SsrA peptide tagging of the E. coli GalE protein (UDP-galactose 4-epimerase). GalE chains are tagged at more than 150 sites, primarily within distinct clusters throughout the C-terminal domain. These tagging sites do not correspond to rare codon clusters and synonymous recoding of the galE gene had little effect on tagging. Moreover, tagging was largely unaffected by perturbations that either stabilize or destabilize the galE transcript. Examination of GalE-thioredoxin (TrxA) fusion proteins showed that the GalE C-terminal domain is no longer tagged when fused to an N-terminal TrxA domain. Conversely, the N-terminus of GalE induced tagging within the fused C-terminal TrxA domain.Conclusions/SignificanceThese findings suggest that translation of the GalE N-terminus induces subsequent tagging of the C-terminal domain. We propose that co-translational maturation of the GalE N-terminal domain influences ribosome pausing and subsequent tmRNA activity.

Structure modeling and comparative genomics for epimerase enzyme (Gal10p).

The Gal10p (UDP-Galactose 4-epimerase) protein is known for regulation of D-galactose metabolism. It catalyzes the inter-conversion between UDPgalactose and UDP-glucose. Knowledge of protein structure, neighboring interacting partners as well as functional residues of the Gal10p is crucial for carry out its function. These problems are still uncovered in case of the Epimerase enzyme. Structure of Epimerase enzyme has already been determined in S.cerevisiae and E.coli, however, no structural information for this protein is available for K.lactis. We used the homology modeling approach to model the structure of Gal10p in K.lactis. Furthermore, functional residues were predicted for modeled Gal10 protein and the strength of interaction between Gal10p and other Gal proteins was carried out by protein -protein interaction studies. The interaction studies revealed that the affinity of Gal10p for other Gal proteins vary in different organisms. Sequence and structure comparison of Epimerase enzyme showed that the orthologs in K.lactis and S.cervisiae are more similar to each other as compared to the ortholog in E.coli .The studies carried by us will help in better understanding of the galactose metabolism. The above studies may be applied to Human Gal10p, where it can help in gaining useful insight into Galactosemia disease.

Homology modeling and molecular dynamics simulation studies of human type 1 3β‐hydroxysteroid dehydrogenase: Toward the understanding of cofactor specificity

The human type 1 isoform of 3β-hydroxysteroid dehydrogenase is a member of short-chain oxidoreductase family that catalyzes the conversion of dehydroepiandrosterone to androstenedione. To compare the molecular events underlying cofactor specificity in the wild-type and D35A/K36R mutant enzymes, molecular dynamics (MD) simulations of fully solvated cofactor-3β-HSD1 (wild-type and mutant) complex are performed. Molecular modeling methods are applied to construct three-dimensional models of cofactor-3β-HSD1 complexes based on Uridine diphosphate (UDP)-galactose 4-epimerase crystal structure from Escherichia coli. The binding mode and binding energy analysis between four different complexes indicate that Asp35 and Lys36 are key residues for the cofactor specificity of 3β-HSD1, which is in agreement with mutagenesis studies results obtained by Thomas et al.8 The MD results also display that the residue Glu41 may be another important residue except Asp35 and Lys36 for the cofactor specificity and that this result needs further mutational experiment for validation.

Crystal structure of UDP‐galactose 4‐epimerase‐like l‐threonine dehydrogenase belonging to the intermediate short‐chain dehydrogenase‐reductase superfamily

The crystal structure of a L-threonine dehydrogenase (L-ThrDH; EC 1.1.1.103) from the psychrophilic bacterium Flavobacterium frigidimaris KUC-1, which shows no sequence similarity to conventional L-ThrDHs, was determined in the presence of NAD and a substrate analog, glycerol. The asymmetric unit consisted of two subunits related by a two-fold rotation axis. Each monomer consisted of a Rossmann-fold domain and a carboxyl-terminal catalytic domain. The overall fold of F. frigidimaris L-ThrDH showed significant similarity to that of UDP-galactose 4-epimerase (GalE); however, structural comparison of the enzyme with E. coli and human GalEs showed clear topological differences in three loops (loop 1, loop 2 and the NAD-binding loop) around the substrate and NAD binding sites. In F. frigidimaris L-ThrDH, loops 1 and 2 insert toward the active site cavity, creating a barrier preventing the binding of UDP-glucose. Alternatively, loop 1 contributes to a unique substrate binding pocket in the F. frigidimaris enzyme. The NAD binding loop, which tightly holds the adenine ribose moiety of NAD in the Escherichia coli and human GalEs, is absent in F. frigidimaris L-ThrDH. Consequently, the cofactor binds to F. frigidimaris L-ThrDH in a reversible manner, unlike its binding to GalE. The substrate binding model suggests that the reaction proceeds through abstraction of the β-hydroxyl hydrogen of L-threonine via either a proton shuttle mechanism driven by Tyr143 and facilitated by Ser118 or direct proton transfer driven by Tyr143. The present structure provides a clear bench mark for distinguishing GalE-like L-ThrDHs from GalEs.

UDP-glucose dehydrogenase from Capra hircus liver: Purification, partial characterization and evaluation as a coupling enzyme in UDP-galactose 4-epimerase assay

Abstract UDP-glucose dehydrogenase from Capra hircus has been purified to homogeneity by salt fractionations, heat treatment and chromatographic steps. It is a homohexamer of about 300 kDa. Though the basic physical and enzymatic properties of the caprine enzyme are comparable to those of the beef liver enzyme, it has lower energy of activation and different entropy and enthalpy for the transition state during catalysis. The caprine enzyme can act suitably as an auxiliary enzyme in the coupled assay system for UDP-galactose 4-epimerase. Enzymes: UDP-Glc DH, UDP-glucose dehydrogenase (EC 1.1.1.22); Epimerase, UDP-galactose 4-epimerase (EC 5.1.3.2).

Practical Preparation ofD-Galactosyl-β1→4-L-rhamnose Employing the Combined Action of Phosphorylases

D-Galactosyl-beta1-->4-L-rhamnose (GalRha) was produced enzymatically from 1.1 M sucrose and 1.0 M L-rhamnose by the concomitant actions of four enzymes (sucrose phosphorylase, UDP-glucose-hexose 1-phosphate uridylyltransferase, UDP-glucose 4-epimerase, and D-galactosyl-beta1-->4-L-rhamnose phosphorylase) in the presence of 1.0 mM UDP-glucose and 30 mM inorganic phosphate. The accumulation of GalRha in 1 liter of the reaction mixture reached 230 g (the reaction yield was 71% from L-rhamnose). Sucrose and fructose in the reaction mixture were removed by yeast treatment, but isolation of GalRha by crystallization after yeast treatment was unsuccessful. Finally, 49 g of GalRha was isolated from part of the reaction mixture with yeast treatment by gel-filtration chromatography.

Generalized epimerase deficiency galactosemia

Galactosemia is caused by inherited deficiencies in one of three enzymes involved in the metabolism of galactose: galactose-1-phosphate uridyltransferase (GALT), galactokinase (GALK), and uridine diphosphate galactose-4-epimerase (GALE). The rarest and most poorly understood form of galactosemia is due to epimerase deficiency. We are reporting such a rarest form of galactosemia presenting with progressively increasing cholestatic jaundice and failure to thrive at one month of age. After confirmation of decreased epimerase level in RBC hemolysate, the patient was put on galactose restricted diet and vitamins supplementation, which reversed the clinical signs as well as altered liver function. Patient is on regular follow-up and now at 15 months of age he has no marked developmental delay.

Galactosemia Diagnosis Gets an Upgrade

Classic galactosemia, a defect of the metabolism of galactose, was one of the earliest defects of intermediary metabolism to be recognized in the mid 20th century(1)(2). The disease is caused by a defect of galactose-1-phosphate uridyltransferase (GALT),1 an enzyme central to the Leloir pathway in which galactose is converted into glucose. Galactose is primarily derived from the lactose content of milk that a newborn receives. The well-described and frequently taught clinical phenotype of an untreated newborn with classic galactosemia begins shortly after the onset of regular milk feeding and includes feeding intolerance with vomiting and diarrhea caused by galactose fermentation in the gut; hepatic failure leading to jaundice; bleeding manifestations; hypoalbuminemia and hypoglycemia; a renal Fanconi syndrome with loss of phosphate, glucose, and amino acids; congenital bilateral cataracts; and susceptibility to fatal gram-negative sepsis (primarily Escherichia coli ). The hepatic and renal manifestations are caused by intracellular accumulation of galactose 1-phosphate, the substrate for GALT. Cataracts are due to an alternative metabolic pathway of galactose in the lens that leads to galactitol accumulation. Treatment is relatively simple. Once the diagnosis is suspected or confirmed, lactose and lactose-containing products are removed from the diet to reduce the metabolic burden, and most of the neonatal toxicities are alleviated. Subsequent to the recognition of GALT deficiency, 2 additional enzymatic defects of galactose metabolism that also caused galactose accumulation were identified. Galactokinase (GALK) converts galactose to galactose 1-phosphate. GALK deficiency leads to failure to generate the cytotoxic galactose 1-phosphate and therefore does not have the potentially fatal hepatic and renal manifestations. GALK-deficient patients are still at high risk for developing cataracts, which may be congenital or may develop later in life(3). The third enzyme is UDP-galactose 4-epimerase (GALE), which converts UDP-galactose to UDP-glucose, which subsequently is converted to glucose or …

Proteomic analysis of common bean seed with storage protein deficiency reveals up-regulation of sulfur-rich proteins and starch and raffinose metabolic enzymes, and down-regulation of the secretory pathway

A deficiency in major seed storage proteins is associated with a nearly two-fold increase in sulfur amino acid content in genetically related lines of common bean (Phaseolus vulgaris). Their mature seed proteome was compared by an approach combining label-free quantification by spectral counting, 2-DE, and analysis of selective extracts. Lack of phaseolin, phytohemagglutinin and arcelin was mainly compensated by increases in legumin, alpha-amylase inhibitors and mannose lectin FRIL. Along with legumin, albumin-2, defensin and albumin-1 were major contributors to the elevated sulfur amino acid content. Coordinate induction of granule-bound starch synthase I, starch synthase II-2 and starch branching enzyme were associated with minor alteration of starch composition, whereas increased levels of UDP-glucose 4-epimerase were correlated with a 30% increase in raffinose content. Induction of cell division cycle protein 48 and ubiquitin suggested enhanced ER-associated degradation. This was not associated with a classical unfolded protein response as the levels of ER HSC70-cognate binding protein were actually reduced in the mutant. Repression of rab1 GTPase was consistent with decreased traffic through the secretory pathway. Collectively, these results have implications for the nutritional quality of common bean, and provide information on the pleiotropic phenotype associated with storage protein deficiency in a dicotyledonous seed.

Catalysis, Structures, and Applications of Carbohydrate Epimerases

More than 20 types of carbohydrate epimerase have been reported to date. Distinctly different enzymes recognize the C1, C2, C3, C4, C5 or C6 position of carbohydrate substrates. They include, for instance, aldose 1-epimerase, N-acetyl-D-glucosamine 2-epimerase, UDP-N-acetylglucosamine 2-epimerase, cellobiose 2-epimerase, D-ribulose-5-phosphate 3-epimerase, L-ribulose-5-phosphate 4-epimerase, UDP-galactose 4-epimerase, dTDP-6-deoxy-D-xylo-4-hexulose 3,5-epimerase, GDP-mannose 3,5-epimerase and ADP-L-glycero-D-mannoheptose 6-epimerase. Their biological properties, catalytic mechanisms and tertiary structures are very diverse. This review focuses on the catalysis, structures and applications of carbohydrate epimerases that have recently been characterized in detail.

Leishmania UDP-sugar Pyrophosphorylase: THE MISSING LINK IN GALACTOSE SALVAGE?

The Leishmania parasite glycocalyx is rich in galactose-containing glycoconjugates that are synthesized by specific glycosyltransferases that use UDP-galactose as a glycosyl donor. UDP-galactose biosynthesis is thought to be predominantly a de novo process involving epimerization of the abundant nucleotide sugar UDP-glucose by the UDP-glucose 4-epimerase, although galactose salvage from the environment has been demonstrated for Leishmania major. Here, we present the characterization of an L. major UDP-sugar pyrophosphorylase able to reversibly activate galactose 1-phosphate into UDP-galactose thus proving the existence of the Isselbacher salvage pathway in this parasite. The ordered bisubstrate mechanism and high affinity of the enzyme for UTP seem to favor the synthesis of nucleotide sugar rather than their pyrophosphorolysis. Although L. major UDP-sugar pyrophosphorylase preferentially activates galactose 1-phosphate and glucose 1-phosphate, the enzyme is able to act on a variety of hexose 1-phosphates as well as pentose 1-phosphates but not hexosamine 1-phosphates and hence presents a broad in vitro specificity. The newly identified enzyme exhibits a low but significant homology with UDP-glucose pyrophosphorylases and conserved in particular is the pyrophosphorylase consensus sequence and residues involved in nucleotide and phosphate binding. Saturation transfer difference NMR spectroscopy experiments confirm the importance of these moieties for substrate binding. The described leishmanial enzyme is closely related to plant UDP-sugar pyrophosphorylases and presents a similar substrate specificity suggesting their common origin.

Characterization of two different types of UDP-glucose/-galactose4-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 uge1Deltagal10Delta 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 uge1Deltagal10Delta 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.