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. (C) 1997 Federation of European Biochemical Societies.