CHEMICAL CHARACTERIZATION OF PH-DEPENDENT STRUCTURAL EPITOPES OF LIPOPOLYSACCHARIDES FROM RHIZOBIUM-LEGUMINOSARUM BIOVAR PHASEOLI

Lipopolysaccharide (LPS) was isolated from free-living Rhizobium legum inosarum bv. phaseoli CE3 cells grown at pH 4.8 (antigenically similar to bacteroid LPS) and compared with that from cells grown at pH 7.2 ( free-living bacteria). Composition analysis revealed that pH 7.2 LPS d iffers from pH 4.8 LPS in that 2,3,4-tri-O-methylfucose is replaced by 2,3-di-O-methylfucose. The amount of 2-O-methylrhamnose is greater in the pH 4.8 LPS than in the pH 7.2 LPS. Analysis of the structural com ponents of LPS (O-chain polysaccharide, core oligosaccharides, and the lipid A) revealed that all the composition differences in the various LPSs occur in the O-chain polysaccharide. These structural variations between pH 4.8 and pH 7.2 LPSs provide a chemical basis for the obser ved lack of cross-reactivity with pH 4.8 LPS of two monoclonal antibod ies, JIM28 and JIM29, raised against free-living bacteria grown at pH 7.2. An LPS preparation isolated from bacteroids contained both 2,3,4- tri-O- and 2,3-di-O-methylfucose residues. This result is consistent w ith the finding that the two monoclonal antibodies react weakly with b acteroid LPS. It is concluded that methylation changes occur on the LP S O-chain of R. leguminosarum bv. phaseoli when the bacteria are grown at low pH and during nodule development.