Characterization and Expression Patterns of UDP-d-Glucuronate Decarboxylase Genes in Barley  

Abstract

UDP-d-glucuronate decarboxylase (EC 4.1.1.35) catalyzes the synthesis of UDP-d-xylose from UDP-d-glucuronate in an essentially irreversible reaction that is believed to commit glycosyl residues to heteroxylan and xyloglucan biosynthesis. Four members of the barley (Hordeum vulgare) UDP-d-glucuronate decarboxylase gene family, designated HvUXS1 to HvUXS4, have been cloned and characterized. Barley HvUXS1 appears to be a cytosolic enzyme, while the others are predicted to be membrane-bound proteins with single transmembrane helices. Heterologous expression of a barley HvUXS1 cDNA in Escherichia coli yields a soluble enzyme that converts UDP-d-glucuronate to UDP-d-xylose, is associated with a single molecule of bound NAD+, and is subject to feedback inhibition by UDP-d-xylose. Quantitative PCR shows that the HvUXS1 mRNA is most abundant among the 4 HvUXS genes, accounting for more than 80% of total HvUXS transcripts in most of the tissues examined. The abundance of HvUXS1 mRNA is 10-fold higher in mature roots and stems than in leaves, developing grains, or floral tissues. Transcriptional activities of HvUXS2 and HvUXS4 genes are relatively high in mature roots, coleoptiles, and stems compared with root tips, leaves, and floral tissues, while HvUXS3 mRNA is low in all tissues. In barley leaf sections, levels of the most abundant mRNA, encoding HvUXS1, reflect the amount of soluble enzymic protein and activity. In selected tissues where HvUXS1 transcript levels are high, cell walls have higher arabinoxylan contents.

The Genetics, Transcriptional Profiles, and Catalytic Properties of UDP-α-d-Xylose 4-Epimerases from Barley

“…9D), and this is consistent with observations that transcript levels of all the HvUXE (Fig. 6) and HvUXS genes (Zhang et al, 2005) were higher in root maturation zones than in root tips. In barley leaves, pentose sugar contents were similar at the tip and base of the leaves but increased in the central regions of leaf segments (Fig.…”

“…9E), which again is consistent with transcript levels of the three HvUXE genes (Fig. 8) and with levels of HvUXS2 and HvUXS3 transcripts (Zhang et al, 2005).…”

“…Additive Percentage None 100 6 4 DTT (2 mM) 150 6 8 EDTA (2 mM) 9 5 6 2 MgCl 2 (1 mM) 101 6 2 NAD + (1 mM) 105 6 2 NADH (1 mM) 9 0 6 3 UMP (2 mM) 9 3 6 1 UDP (2 mM) 1 5 6 1 UTP (2 mM) 5 4 6 3 thesis of pentose-containing wall polysaccharides (Reiter and Vanzin, 2001;Seifert, 2004;Zhang et al, 2005). Furthermore, it is believed that a UDP-Ara mutase contributes to the composition of this pool through the conversion of UDP-Arap to UDP-Araf (Konishi et al, 2007).…”

“…Because of the importance of UXS in diverting sugar nucleotides into the "pentose pool" (Zhang et al, 2005), the abundance of HvUXS transcripts was also monitored in these experiments. The HvUXS1 gene is the most highly transcribed of the four HvUXS genes in the organs of barley so far examined (Zhang et al, 2005).…”

“…Following the epimerization of UDP-GlcA to UDP-D-GalUA, the latter acts as a sugar donor for pectic polysaccharide synthesis (Reiter and Vanzin, 2001;Seifert, 2004). The UDP-GlcA decarboxylases, also known as UDP-a-D-xylopyranose synthases (UXS), convert UDP-GlcA to UDP-Xyl (Zhang et al, 2005), which acts as the xylosyl donor not only for the biosynthesis of arabinoxylans but also for xyloglucans.…”