Functional cloning and characterization of a UDP-glucuronic acid decarboxylase: The pathogenic fungus Cryptococcus neoformans elucidates UDP-xylose synthesis

UDP-xylose is a sugar donor required for the synthesis of diverse and impor tant glycan structures in animals, plants, fungi, and bacteria. Xylose-cont aining glycans are particularly abundant in plants and in the polysaccharid e capsule that is the major virulence factor of the pathogenic fungus Crypt ococcus neoformans. Biosynthesis of UDP-xylose is mediated by UDP-glucuroni c acid decarboxylase, which converts UDP-glucuronic acid to UDP-xylose. Alt hough this enzymatic activity was described over 40 years ago it has never been fully purified, and the gene encoding it has not been identified. We u sed homology to a bacterial gene, hypothesized to encode a related function , to identify a cryptococcal sequence as putatively encoding a UDP-glucuron ic acid decarboxylase. A soluble 47-kDa protein derived from bacteria expre ssing the C. neoformans gene catalyzed conversion of UDP-glucuronic acid to UDP-xylose, as confirmed by NMR analysis. NADH, UDP, and UDP-xylose inhibi t the activity. Close homologs of the cryptococcal gene, which we termed UX S1, appear in genome sequence data from organisms ranging from bacteria to humans.