A bifunctional epimerase-reductase acts downstream of the MUR1 gene product and completes the de novo synthesis of GDP-L-fucose in Arabidopsis

L-Fucose is a monosaccharide found as a component of glycoproteins and cell wall polysaccharides in higher plants. The MUR1 gene of Arabidopsis thaliana encodes a GDP-D-mannose 4,6-dehydratase catalyzing the first step in the de novo synthesis of GDP-L-fucose from GDP-D-mannose (Bonin et al. 1997, Proc. Natl Acad. Sci. USA, 94, 2085-2090). Plant genes encoding the subsequent steps in L-fucose synthesis (3,5-epimerization and 4-reduction) have not been described previously. Based on sequence similarities to a bacterial gene involved in capsule synthesis we have cloned a gene from Arabidopsis, now designated GER1, which encodes a bifunctional 3, 5-epimerase-4-reductase in L-fucose synthesis. The combined action of the MUR1 and GER1 gene products converts GDP-D-mannose to GDP-L-fucose in vitro demonstrating that this entire nucleotide-sugar interconversion pathway could be reconstituted using plant genes expressed in Escherichia coli. In vitro assays indicated that the GER1 protein does not act as a GDP-D-mannose 3, 5-epimerase, an enzymatic activity involved in the de novo synthesis of GDP-L-galactose and L-ascorbic acid. Similarly, L-ascorbate levels in GER1 antisense plants were unchanged indicating that GDP-D-mannose 3,5-epimerase is encoded by a separate gene.