Nucleotide sugars serve as sugar donors for the biosynthesis of various cell components including cell wall, glycoproteins, and small molecules. Among them, uridine diphosphate (UDP)-glucose is one of main nucleotide sugars that serve as a substrate for the synthesis of other UDP-sugars. UDP-sugar pyrophosphorylase (USPase) mediates the formation of UDP-glucose from uridine triphosphate (UTP) and glucose-1-phosphate. A USPase, PdUSPase was cloned from Populus deltoids and expressed in Escherichia coli as glutathione S-transferase fusion protein. The purified recombinant PdUSPase catalyzed the reaction for the formation of UDP-glucose from glucose-1-phosphate and UTP, and for the formation of UDP-galactose from galactose-1-phosphate and UTP. However, the enzyme did not show any activity toward mannose-1-phosphate and UTP. These results indicate that PdUSPase belonging in UGPase A in phylogenetic analysis is the first UDP-glucose synthesizing enzyme showing a discrepancy between phylogenetic analysis and substrate range. E. coli complementation was also carried out to confirm the function of PdUSPase using E. coli galU mutant, which was mutated in UTP glucose-1-phosphate uridyltransferase. The galU mutant was transformed with the PdUGTase gene and a flavonoid glucosyl-transferase gene, AtUGT78D2. The resulting transformant was able to convert quercetin into quercetin 3-O-glucose similarly to that by the wild type E. coli strain harboring AtUGT78D2. These results indicated that PdUSPase catalyzed the formation of UDP-glucose from UTP and glucose-1-phosphate.