Abstract
d-Galacturonic acid (GalA) is an important component of GalA-containing polysaccharides in Ornithogalum caudatum. The incorporation of GalA into these polysaccharides from UDP-d-galacturonic acid (UDP-GalA) was reasonably known. However, the cDNAs involved in the biosynthesis of UDP-GalA were still unknown. In the present investigation, one candidate UDP-d-glucuronic acid 4-epimerase (UGlcAE) family with three members was isolated from O. caudatum based on RNA-Seq data. Bioinformatics analyses indicated all of the three isoforms, designated as OcUGlcAE1~3, were members of short-chain dehydrogenases/reductases (SDRs) and shared two conserved motifs. The three full-length cDNAs were then transformed to Pichia pastoris GS115 for heterologous expression. Data revealed both the supernatant and microsomal fractions from the recombinant P. pastoris expressing OcUGlcAE3 can interconvert UDP-GalA and UDP-d-glucuronic acid (UDP-GlcA), while the other two OcUGlcAEs had no activity on UDP-GlcA and UDP-GalA. Furthermore, expression analyses of the three epimerases in varied tissues of O. caudatum were performed by real-time quantitative PCR (RT-qPCR). Results indicated OcUGlcAE3, together with the other two OcUGlcAE-like genes, was root-specific, displaying highest expression in roots. OcUGlcAE3 was UDP-d-glucuronic acid 4-epimerase and thus deemed to be involved in the biosynthesis of root polysaccharides. Moreover, OcUGlcAE3 was proposed to be environmentally induced.
Highlights
UDP-D-glucuronic acid 4-epimerase, a very specific membrane-bound 4-epimerase, is able to catalyze the interconversion of UDP-GlcA and UDP-Galacturonic acid (GalA) (Scheme 1) [1,2,3]
Data revealed both the supernatant and microsomal fractions from the recombinant P. pastoris expressing OcUGlcAE3 can interconvert UDP-GalA and UDP-D-glucuronic acid (UDP-GlcA), while the other two OcUGlcAEs had no activity on UDP-GlcA and UDP-GalA
Expression analyses of the three epimerases in varied tissues of O. caudatum were performed by real-time quantitative PCR (RT-qPCR)
Summary
UDP-D-glucuronic acid 4-epimerase, a very specific membrane-bound 4-epimerase, is able to catalyze the interconversion of UDP-GlcA and UDP-GalA (Scheme 1) [1,2,3]. UDP-GalA, the activated nucleotide sugar form of GalA, can serve as the precursor of various bacterial polysaccharides and plant pectins [3,4,5]. UDP-GalA is the precursor of health-promoting galacturonides, providing GalA component for the synthesis of galacturonides [6,7,8,9]. By this token, UGlcAE played crucial role in the synthesis of GalA-containing molecules with potent activities. UGlcAE activities had been detected in diverse prokaryotes including Bacillus cereus [4], Klebsiella pneumonia [10], Sinorhizobium meliloti [11], Escherichia coli [12] and Streptococcus pneumonia [13]. Relatively less information was available on cDNA isolation and functional characterization of genes encoding
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