Investigation of Lipopolysaccharides from Sinorhizobium meliloti SKHM1-188 and Two of Its Mutants with Decreased Nodulation Competitiveness

Abstract

A comparative study of the lipopolysaccharides (LPS) isolated from Sinorhizobium meliloti SKHM1-188 and two of its LPS mutants (Tb29 and Ts22) with sharply decreased nodulation competitiveness was conducted. Polyacrylamide gel electrophoresis with sodium dodecyl sulfate revealed two forms of LPS in all three strains: a higher molecular weight LPS1, containing O-polysaccharide (O-PS), and a lower molecular weight LPS2, without O-PS. However, the LPS1 content in mutants was significantly smaller than in the parent strain. The LPS of the strains studied contained glucose, galactose, mannose, xylose, three nonidentified sugars (X ₁ (TGlc 0.53), X ₂ (TGlc 0.47), and X ₃ (TGlc 0.43)), glucosamine, and ethanolamine, while the LPS of S. meliloti SKHM1-188 additionally contained galactosamine, glucuronic and galacturonic acids, and 2-keto-3-deoxyoctulosonic acid (KDO), as well as such fatty acids as 3-OH C14:0, 3-OH C15:0, 3-OH C16:0, 3-OH C18:0, nonidentified hydroxy X (T_{3-OH C14:0} 1.33), C18:0, and unsaturated C18:1 fatty acids. The LPS of both mutants were similar in the component composition but differed from the LPS of the parent strain by lower X ₂, X ₃, and 3-OH C14:0 contents and higher KDO, C18:0, and hydroxy X contents. The LPS of all the strains were subjected to mild hydrolysis with 1% acetic acid and fractionated on a column with Sephadex G-25. The higher molecular weight fractions (2500–4000 Da) contained a set of sugars typical of intact LPS and, supposedly, corresponded to the LPS polysaccharide portion (PS1). In the lower molecular weight fractions (600–770 Da, PS2), glucose and uronic acids were the major components; galactose, mannose, and X ₁ were present in smaller amounts. The PS1/PS2 ratio for the two mutants was significantly lower than for strain SKHM1-188. The data obtained show that the amount of O-PS–containing molecules (LPS1) in the heterogeneous lipopolysaccharide complex of the mutants was smaller than in the SKHM1-188 LPS; this increases the hydrophobicity of the cell surface of the mutant bacteria, which supposedly contributes to their nonspecific adhesion to the roots of the host plant, thus decreasing their nodulation competitiveness.