Lipopolysaccharide biosynthesis in Rhizobium leguminosarum. Novel enzymes that process precursors containing 3-deoxy-D-manno-octulosonic acid.

The lipopolysaccharide of Rhizobium leguminosarum differs from that of other Gram-negative organisms. R. leguminosarum lipid A lacks phosphate groups, but it contains a galacturonic acid residue at the 4'-position and an aminogluconate moiety in place of the usual glucosamine 1-phosphate unit. R. leguminosarum lipid A is esterified with a peculiar long chain fatty acid, 27-hydroxyoctacosanoate, not found in enteric Gram-negative bacteria, and the inner core of R. leguminosarum contains mannose and galactose in place of heptose. Despite these differences, the biosynthesis of R. leguminosarum lipid A is initiated by the same seven enzyme pathway as in Escherichia coli (Raetz, C. R. H. (1993) J. Bacteriol. 175, 5745-5753) to form the phosphorylated precursor, (Kdo)2-lipid IVA, which is then processed differently. We now describe several novel Rhizobium-specific enzymes that recognize and modify (Kdo)2-lipid IVA. The 1- and 4'-phosphatases were detected using (Kdo)2-[1-32P]-lipid IVA and (Kdo)2-[4'-32P]-lipid IVA, respectively, as shown by release of 32Pi. In the presence of GDP-mannose and/or UDP-galactose, membranes of R. leguminosarum first transferred mannose and then galactose to (Kdo)2-[4'-32P]-lipid IVA. In addition, at least two hydrophobic metabolites were generated from (Kdo)2-[4'-32P]-lipid IVA in a manner that was dependent upon both membranes and a cytosolic factor from R. leguminosarum. These compounds are attributed to novel acylations of (Kdo)2-[4'-32P]-lipid IVA. E. coli membranes and cytosol did not catalyze any of the unique reactions detected in R. leguminosarum extracts. Our findings establish the conservation and versatility of (Kdo)2-lipid IVA as a lipid A precursor in bacteria.