The nodulation gene nolK of Azorhizobium caulinodans is involved in the formation of GDP‐fucose from GDP‐mannose

The nolK gene of Azorhizobium caulinodans is essential for the incorporation of a fucosyl group in Nod factors. A NAD(P)-binding site is present in the NolK amino acid sequence and the gene is homologous to Escherichia coli genes, presumably involved in GDP-fucose synthesis. Protein extracts of A. caulinodans, overexpressing nolK, have an enzyme activity that synthesizes GDP-fucose from GDP-mannose. nolK most probably encodes a 4-reductase performing the last step in GDP-fucose synthesis. Wild-type A. caulinodans produces a population of fucosylated and non-fucosylated molecules but the nolK-overexpressing strain produces only fucosylated Nod factors. Thus, the production of activated fucosyl donors is a rate-limiting step in Nod factor fucosylation.