Molecular cloning and characterization of a UDP- glucose-4- epimerase gene (galE) and its expression in pea tissues

We cloned a cDNA encoding for the enzyme UDP-glucase-4-epimerase (EC 5.1.3.2; UDP-galactose-4-epimerase or 'galactowaldenase') in pea (Pisum sativum L. cv. Wando). This enzyme functions in the interconversion of UDP-glucose and UDP-galactose. The open reading frame within this cDNA contains 350 amino acid residues and the polypeptide product has a predicted molecular mass of 38 994 Da. The pea enzyme exhibits extensive homology with the deduced sequences of other prokaryotic and eukaryotic UDP-glucose-4-epimerases. Northern analysis of expression of the UDP-glucose-4-epimerase gene (galE) in pea tissues revealed 3- and 5-fold higher mRNA levels in the leaves and roots, respectively, of 3-week old seedlings, as compared with 4-day old seedlings. GalE transcript levels did not differ significantly in the roots and leaves of light-grown versus etiolated 4-day old seedlings. Increased galE expression in the roots of older seedlings may be correlated with the copious secretion of mucigel at the root tip, as increased availability of the UDP-galactose precursor may be required to support the synthesis of this pectinaceous extracellular matrix. (C) Elsevier, Paris.