Characterization of polysaccharides of Rhizobium meliloti exo mutants that form ineffective nodules

Abstract

Mutants of Rhizobium meliloti SU47 with defects in the production of the Calcofluor-binding exopolysaccharide succinoglycan failed to gain entry into alfalfa root nodules. In order to define better the polysaccharide phenotypes of these exo mutants, we analyzed the periplasmic oligosaccharide cyclic (1-2)-p-D-glucan and lipopolysaccharide (LPS) in representative mutants. The exoC mutant lacked the glucan and had abnormal LPS which appeared to lack a substantial portion of the 0 side chain. The exoB mutant had a spectrum of LPS species which differed from those of both the wild-type parental strain and the exoC mutant. The presence of the glucan and normal LPS in the exoA, exoD, exoF, and exoH mutants eliminated defects in these carbohydrates as explanations for the nodule entry defects of these mutants. We also assayed for high-and low-molecular-weight succinoglycans. All of the exo mutants except exoD and exoH completely lacked both forms. For the Calcofluor-dim exoD mutant, the distribution of high-and low-molecular-weight forms depended on the growth medium. The haloless exoH mutant produced high-molecular-weight and only a trace of low-molecular-weight succinoglycan; the succinyl modification was missing, as was expected from the results of previous studies. The implications of these observations with regard to nodule entry are discussed.

Regulation of Rhizobium meliloti exo genes in free-living cells and in planta examined by using TnphoA fusions

“…It is perhaps not surprising that exoB is not regulated by exoR and exoS because exoB mutants are also defective for the production of a second exopolysaccharide, EPS II (13,41), and show lipopolysaccharide alterations (4,18). Recently it has been shown that the exoB gene encodes UDP-galactose epimerase, which converts UDP-glucose to UDP-galactose (3a olism [1], and therefore exoB mutants are not galactose sensitive.)…”

An exoB mutant of Rhizobium sp. is effective in indeterminate nodules of Hedysarum coronarium

“…Furthermore, mutants had the same nodulation kinetic and fixed nitrogen at the same (Leigh & Lee, 1988;Canter Cremers e t al., 1990). The mutant is not mucoid and the residual EPS, isolated using the method described by Dielbold & Noel (1989), lacks galactose residues and the acidic component (Fig.…”

“…It has been reported that the exoB gene is necessary for successful nodulation on legumes that develop an indeterminate type of nodule (Leigh e t al., 1985;Leigh & Lee, 1988;Canter Cremers etal., 1990), but does not seem to be essential for nodulation on legumes with a determinate type of nodule (Parniske e t al., 1993). However, we report in this paper an exoB mutant of Rhixobitlm sp.…”

“…They are defective in the synthesis of both EPS and LPS and induce empty ineffective nodules (Leigh e t al., 1985 ;Leigh & Lee, 1988) or only some root hair deformation and rare abortive infection threads (Canter Cremers e t al., 1990 * Resistance to ampicillin (Ap'), chloramphenicol (Cm'), kanamycin (Km'), rifampicin (Rif), streptomycin (Sm'), tetracycline (Tc') and trimethoprim (Tp').…”

Cloning and analysis of a 35.3-kilobase DNA region involved in exopolysaccharide production by Xanthomonas campestris pv. campestris

“…In general, one has to be aware of the fact that particular mutations may have pleiotropic effects on the bacterial envelope components, because EPSs and lipopolysaccharides are partially composed of the same sugar subunits; therefore, the same precursors are utilized in the synthesis of both compounds (8,30). Pleiotropic mutants have already been isolated from Xanthomonas, Rhizobium, and Pseudomonas strains (e.g., 13,19,29,30). In this paper, we describe the cloning and characterization of a new gene region involved in xanthan gum synthesis.…”