LIPID-A BIOSYNTHESIS IN RHIZOBIUM-LEGUMINOSARUM - ROLE OF A 2-KETO-3-DEOXYOCTULOSONATE-ACTIVATED 4'-PHOSPHATASE

Lipid A from several strains of the N-2-fixing bacterium Rhizobium leg uminosarum displays significant structural differences from Escherichi a coli lipid A, one of which is the complete absence of phosphate grou ps. However, the first seven enzymes of E. coli lipid A biosynthesis, leading from UDP-GlcNAc to the phosphorylated intermediate, 2-keto-3-d eoxyoctulosonate (Kdo(2))-lipid IVA, are present in R. leguminosarum. We now describe a membrane-bound phosphatase in R. leguminosarum extra cts that removes the 4' phosphate of Kdo(2)-lipid IVA. The 4' phosphat ase is selective for substrates containing the Kdo domain. It is prese nt in extracts of R. leguminosarum biovars phaseoli, viciae, and trifo lii but is not detectable in E. coli and Rhizobium meliloti. A nodulat ion-defective strain (24AR) of R. leguminosarum biovar trifolii, known to contain a 4' phosphate residue on its lipid A, also lacks measurab le 4' phosphatase activity. The Kdo-dependent 4' phosphatase appears t o be a key reaction in a pathway for generating phosphate-deficient li pid A.