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The article discusses a study which demonstrated that increased exopolysaccharide succinoglycan production is not detrimental to symbiotic development but enhances the symbiotic productivity of Sinorhizobium meliloti 1021 with the host plant Medicago truncatula cv. ]emalong A17. The study showed that increased succinoglycan production was engineered by overexpression of the exoYgene. It also proposed several possible roles for succinoglycan in infection thread initiation and development.

The patB gene product is required for growth and survival of the filamentous cyanobacterium Anabaena sp. strain PCC 7120 in the absence of combined nitrogen. A patB::gfp fusion demonstrated that this gene is expressed exclusively in heterocysts.patB mutants have a normal initial pattern of heterocyst spacing along the filament but differentiate excess heterocysts after several days in the absence of combined nitrogen. Expression of hetR and patS, two critical regulators of the heterocyst development cascade, are normal for patB mutants, indicating that patB acts downstream of them in the differentiation pathway. A patB deletion mutant suffers an almost complete cessation of growth and nitrogen fixation within 24 h of combined nitrogen removal. In contrast, a new PatB mutant that is defective in its N-terminal ferredoxin domain, or a previously described mutant that has a frameshift removing its C-terminal helix-turn-helix domain, grows very slowly and differentiates multiple contiguous heterocysts under nitrogen-deficient conditions. [ABSTRACT FROM AUTHOR]

Identifies cytochrome c oxidase operon in the nitrogen-fixing cyanobacterium Anabaena specie strain PCC 7120. Correlation between heterocysts and nitrogenases functions; Differentiation of heterocysts by Anabaena strain; Expression of operons in the proheterocysts.

The type IV secretion system (T4SS) of the plant intracellular symbiont Sinorhizobium meliloti 1021 is required for conjugal transfer of DNA. However, it is not required for host invasion and persistence, unlike the T4SSs of closely related mammalian intracellular pathogens. A comparison of the requirement for a bacterial T4SS in plant versus animal host invasion suggests an important difference in the intracellular niches occupied by these bacteria. [ABSTRACT FROM AUTHOR]

Although the role that lipopolysaccharide (LPS) plays in the symbiosis between Sinorhizobium meliloti and alfalfa has been studied for over a decade, its function in this process remains controversial and poorly understood. This is largely due to a lack of mutants affected by its synthesis. In one of the definitive studies concerning this issue, Clover et al. (R. H. Clover, J. Kieber, and E. R. Signer, J. Bacteriol. 171:3961-3967, 1989) identified a series of mutants with putative LPS defects, judged them to be symbiotically proficient on Medicago sativa, and concluded that LPS might not have a symbiotic function in S. meliloti. The mutations in these strains were never characterized at the molecular level nor was the LPS from most of them analyzed. We have transduced these mutations from the Rm2011 background from which they were originally isolated into the sequenced strain Rml021 and have characterized the resulting strains in greater detail. We found the LPS from these mutants to display a striking complexity of phenotypes on polyacrylamide electrophoresis gels, including additional rough LPS bands and alterations in the molecular weight distribution of the smooth LPS. We found that some of the mutants contain insertions in genes that are predicted to be involved in the synthesis of carbohydrate components of LPS, including ddhB, lpsB, lpsC, and lpsE. The majority, however, code for proteins predicted to be involved in a wide variety of functions not previously recognized to play a role in LPS synthesis, including a possible transcription elongation factor (GreA), a possible queuine synthesis protein, and a possible chemotaxis protein. Furthermore, using more extensive assays, we have found that most of these strains have symbiotic deficiencies. These results support more recent findings that alterations in LPS structure can affect the ability of S. meliloti to form an effective symbiosis. [ABSTRACT FROM AUTHOR]

Rhizobia are nitrogen-fixing bacteria that establish endosymbiotic associations with legumes. Nodule formation depends on various bacterial carbohydrates, including lipopolysaccharides, K-antigens, and exopolysaccharides (EPS). An acidic EPS from Rhizobium sp. strain NGR234 consists of glucosyl (Glc), galactosyl (Gal), glucuronosyl (GlcA), and 4,6-pyruvylated galactosyl (PvGal) residues with β-1,3, β-1,4, β-1,6, α-1,3, and α-1,4 glycoside linkages. Here we examined the role of NGR234 genes in the synthesis of EPS. Deletions within the exoF, exoL, exoP, exoQ, and exoY genes suppressed accumulation of EPS in bacterial supernatants, a finding that was confirmed by chemical analyses. The data suggest that the repeating subunits of EPS are assembled by an ExoQ/ExoP/ExoF-dependent mechanism, which is related to the Wzy polymerization system of group 1 capsular polysaccharides in Escherichia coli. Mutation of exoK (NGR Ω exoK), which encodes a putative glycanase, resulted in the absence of low-molecular-weight forms of EPS. Analysis of the extracellular carbohydrates revealed that NGR Ω exoK is unable to accumulate exo-oligosaccharides (EOSs), which are O-acetylated nonasaccharide subunits of EPS having the formula Gal(Glc)5(GlcA)2PvGal. When used as inoculants, both the exo-deficient mutants and NGR Ω exoK were unable to form nitrogen-fixing nodules on some hosts (e.g., Albizia lebbeck and Leucaena leucocephala), but they were able to form nitrogen-fixing nodules on other hosts (e.g., Vigna unguiculata). EOSs of the parent strain were biologically active at very low levels (yield in culture supernatants, ∼50 µg per liter). Thus, NGR234 produces symbiotically active EOSs by enzymatic degradation of EPS, using the extracellular endo-β-1,4-glycanase encoded by exoK (glycoside hydrolase family 16). We propose that the derived EOSs (and not EPS) are bacterial components that play a crucial role in nodule formation in various legumes. [ABSTRACT FROM AUTHOR]