The outer membrane protein TolC from Sinorhizobium meliloti affects protein secretion, polysaccharide biosynthesis, antimicrobial resistance, and symbiosis.

Sinorhizobium meliloti is capable of establishing a symbiotic nitrogen fixation relationship with Medicago sativa. During this process, it must cope with diverse environments and has evolved different types of transport systems that help its propagation in the plant roots. TolC protein family members are the outer-membrane components of several transport systems involved in the export of diverse molecules, playing an important role in bacterial survival.

[TolC mutant of Sinorhizobium meliloti strain SKHM1-188 fails to establish effective symbiosis with alfalfa].

The TolC mutant Tr63 of Sinorhizobium meliloti was generated by random Tn5 mutagenesis in the effective strain SKhM1-188. The mutant did not produce fluorescent halos in UV light on the LB medium containing calcofluor white, which suggests that modification occurred in the production of exopolysaccharide EPS1. Mutant Tr63 also manifested nonmucoidness both on minimal and low-phosphate MOPS media, and this was most likely connected with the absence of the second exopolysaccharide of S.

Overproduction and increased molecular weight account for the symbiotic activity of the rkpZ-modified K polysaccharide from Sinorhizobium meliloti Rm1021.

K polysaccharides (KPSs) of Sinorhizobium meliloti strains are strain-specific surface polysaccharides analogous to the group II K antigens of Escherichia coli. The K(R)5 antigen of strain AK631 is a highly polymerized disaccharide of pseudaminic and glucuronic acids. During invasion of host plants, this K antigen is able to replace the structurally different exopolysaccharide succinoglycan (EPS I) and promotes the formation of a nitrogen-fixing (Fix(+)) symbiosis.

[Identification of Sinorhizobium meliloti genes influencing synthesis of surface polysaccharides and competitiveness].

A new approach to isolating mutants with altered composition of capsular polysaccharides (CPS) and lipopolysaccharides (LPS) in nodule bacteria of alfalfa Sinorhizobium meliloti based on analysis of their respiratory activity was proposed. Random Tn5-mob mutants of symbiotically effective strain SKhM1-105 were tested for slime-production ability and coloration on diagnostic media containing the indicator of reducing equivalents, triphenyltetrasolium chloride (TTC), the inhibitor of respiratory activity, 2-methyl-4-chlorphenoxybutyric acid and Congo Red, the stain for LPS and exopolysaccharides (EPS). Electrophoretic analysis (SDS-PAGE) of polysaccharides in seven mutants, markedly differing from the parental strain with respect to their growth on diagnostic media demonstrated that (1) the production of CPS was either decreased (in mutants T64 and T835) or blocked (in T71, T94, T124, T134, and T170); (2) the R form of LPS had changed mobility (in T134); (3) the S form of LPS contained only one component instead of two (T71).

Recent advances in studies on structure and symbiosis-related function of rhizobial K-antigens and lipopolysaccharides.

Exopolysaccharides (EPSs) and K polysaccharides (K-antigens, capsular polysaccharides, or KPSs) are important for the recognition of the symbiotic partner and the infection process, whereas lipopolysaccharides (LPSs) may function at a later stage of symbiosis. Recently, considerable progress has been made in the structural investigation of rhizobial K-antigens and LPSs. This structural data, together with the availability of more and more mutant data, allows new insights into the structure-function relationships of surface polysaccharides and the mode of their action on host cells.