Variations in outer-membrane characteristics of two stem-nodulating bacteria of Sesbania rostrata and its role in tolerance towards diverse stress.

Outer-membrane characteristics may determine the survivability of rhizobia under diverse abiotic and biotic stresses. Therefore, the role of lipopolysaccharides (LPS) and membrane proteins of two stem-nodulating bacteria of Sesbania rostrata (Azorhizobium caulinodans ORS571 and Rhizobium sp. WE7) in determining tolerance towards abiotic and biotic stresses (hydrophobics and phages) was investigated. Outer-membrane characteristics (LPS and membrane-protein profiles) of ORS571, WE7 and thirteen standard strains were distinct. ORS571 and WE7 also showed susceptibility towards morphologically distinct phages, i.e., ACSR16 (short-tailed) and WESR29 (long-tailed), respectively. ORS571 and WE7 were tolerant to hydrophobic compounds (triton X-100, rifampicin, crystal violet and deoxycholate). To ascertain the role of outer membrane characteristics in stress tolerance, phage-resistant transconjugant mutants of ORS571 (ORS571-M8 and ORS571-M20) and WE7 (WE7-M9) were developed. LPS- and membrane-protein profiles of mutants differed from that of respective wild types (ORS571 and WE7). In in vitro assay, phages got adsorbed onto purified LPS-membrane protein fractions of wild types. Phages did not adsorb onto membrane fraction of mutants and standard strains. Mutant with reduced expression of LPS (ORS571-M20 and WE7-M9) showed reduced tolerance towards hydrophobics. However, the tolerance was unaffected in mutant (ORS571-M8) where expression of LPS was not reduced but pattern was different. The tolerance level of mutants towards hydrophobics varied with the expression of LPS, whereas the specificity towards phages is correlated with the specific LPS pattern.

Phage specificity and lipopolysaccarides of stem- and root-nodulating bacteria (Azorhizobium caulinodans, Sinorhizobium spp., and Rhizobium spp.) of Sesbania spp.

Phage susceptibility pattern and its correlation with lipopolysaccharide (LPS) and plasmid profiles may help in understanding the phenotypic and genotypic diversity among highly promiscuous group of rhizobia nodulating Sesbania spp.; 43 phages were from two stem-nodulating bacteria of S. rostrata and 16 phages were from root-nodulating bacteria of S. sesban, S. aegyptica and S. rostrata. Phage susceptibility pattern of 38 Sesbania nodulating bacteria was correlated with their LPS rather than plasmid profiles. Different species of bacteria (A. caulinodans- ORS571, SRS1-3 and Sinorhizobium saheli- SRR907, SRR912) showing distinct LPS subtypes were susceptible to different group of phages. Phages could also discriminate the strains of Si. saheli (SSR312, SAR610) possessing distinct LPS subtypes. Phages of Si. meliloti (SSR302) were strain-specific. All the strains of R. huautlense having incomplete LPS (insignificant O-chain) were phage-resistant. In in vitro assay, 100% of the phages were adsorbed to LPS of indicator bacterium or its closely related strain(s) only. These observations suggest the significance of LPS in phage specificity of Sesbania nodulating rhizobia. Highly specific phages may serve as biological marker for monitoring the susceptible bacterial strains in culture collections and environment.

Diversity in a promiscuous group of rhizobia from three Sesbania spp. colonizing ecologically distinct habitats of the semi-arid Delhi region.

Sesbania-rhizobia associations have immense significance in soil amelioration programs for diverse habitats. Diversity in symbiotic properties, LPS profiles, Sym plasmid and rhizobiophage sensitivity of 28 root- and stem-nodulating bacterial isolates of three Sesbania species (S. sesban, S. aegyptica and S. rostrata) inhabiting six ecologically distinct sites of semi-arid Delhi region was analyzed. The isolates were highly promiscuous among the symbiotic partners (Sesbania spp.). The root nodules formed by all the isolates were morphologically similar but they differed in their symbiotic efficiency and effectiveness. 16S rDNA sequence analyses revealed that root nodule isolates of sesbanias belong to diverse rhizobial taxa (Sinorhizobium saheli, S. meliloti, Rhizobium huautlense) whereas stem-nodule isolates were strictly Azorhizobium caulinodans. Sinorhizobium spp. seem to dominate as microsymbiont partner of Sesbania in the Delhi region. The genetic diversity revealed by cluster analyses based on NPC-PCR reflects sorting of isolates across the ecological gradient. Parallel diversity was also observed in the grouping based on LPS profiles and sym plasmid (NPC-PCR). Segregation of different rhizobial taxa into distinct types/clusters based on LPS and NPC-PCR analyses suggest its significance in the circumscription of the taxa. However, subtypes and subclusters showed their sorting across the ecological gradients. Sesbania rhizobia showed extremely high specificity to rhizobiophages. Enormous diversity in LPS profiles and high specificity of rhizobiophages might be the result of environmental selection pressures operating in ecologically distinct habitats. The ability of sesbanias to enter into effective symbioses with different rhizobial taxa and colonize diverse habitats with various biotic and abiotic stresses appears to contribute to its wide ecological amplitude.

Exploring the mechanisms of molecular recognition by flavins.

The design and development of chemical models for enzymes depends on the fundamental principles observed in biological systems. Molecular recognition of flavins by various receptors has attracted attention due to their applications as chemical models for flavoenzymes. The area of molecular recognition is being investigated through research at the interface of chemistry and biology. In this review, the literature has been surveyed to provide comprehensive coverage of the synthetic methodology of different flavins and receptors and their molecular-recognition studies. Various applications of flavin-receptor complexes have also been highlighted.