A Comparison of the Surface Polysaccharides from Rhizobium leguminosarum 128C53 sm^rrif^r with the Surface Polysaccharides from Its Exo⁻¹ Mutant

Abstract

The surface polysaccharides of Rhizobium leguminosarum 128C53 sm^rrif^r (parent) and its exo⁻¹ mutant were isolated and characterized. The parent carries out normal symbiosis with its host, pea, while the exo⁻¹ mutant does not nodulate the pea. The following observations were made. (a) The parent produces lipopolysaccharide (LPS), typical acidic extracellular polysaccharide (EPS), and three additional polysaccharides, PS1, PS2, and PS3. The PS1 and PS2 fractions are likely to be the capsular polysaccharide (CPS) and are identical in composition to the EPS. The PS3 fraction is a small-molecular-weight glucan. (b) The exo⁻¹ mutant produces LPS, EPS, and a PS3 fraction, but does not produce significant amounts of either PS1 or PS2. The LPS from the exo⁻¹ mutant appears to be identical to the parental LPS. Analysis of the EPS from exo⁻¹ shows that it consists of two polysaccharides. One polysaccharide is identical to the LPS and comprises 70% of the exo⁻¹ EPS. The second polysaccharide is identical to the exo⁻¹ PS3 and comprises 30% of the exo⁻¹ EPS. This result shows that the exo⁻¹ mutant does not produce any of the typical acidic parental EPS and that the major polysaccharide released into the media by the exo⁻¹ mutant is intact LPS. The exo⁻¹ mutant PS3 fraction was found to contain two polysaccharides, PS3-1 and PS3-2. The PS3-2 polysaccharide is identical to the parental PS3 described above. The PS3-1 polysaccharide has a composition similar to the polysaccharide portion of the LPS. This result suggests that the exo⁻¹ mutant produces LPS polysaccharide fragments. These LPS polysaccharide fragments are not produced by the parent strain.