Inositol Catabolism, a Key Pathway in Sinorhizobium meliloti for Competitive Host Nodulation

Abstract

The nitrogen-fixing symbiont of alfalfa, Sinorhizobium meliloti, is able to use myo-inositol as the sole carbon source. Putative inositol catabolism genes (iolA and iolRCDEB) have been identified in the S. meliloti genome based on their similarities with the Bacillus subtilis iol genes. In this study, functional mutational analysis revealed that the iolA and iolCDEB genes are required for growth not only with the myo-isomer but also for growth with scyllo-and D-chiro-inositol as the sole carbon source. An additional, hypothetical dehydrogenase of the IdhA/MocA/GFO family encoded by the smc01163 gene was found to be essential for growth with scyllo-inositol, whereas the idhA-encoded myo-inositol dehydrogenase was responsible for the oxidation of D-chiro-inositol. The putative regulatory iolR gene, located upstream of iolCDEB, encodes a repressor of the iol genes, negatively regulating the activity of the myo-and the scyllo-inositol dehydrogenases. Mutants with insertions in the iolA, smc01163, and individual iolRCDE genes could not compete against the wild type in a nodule occupancy assay on alfalfa plants. Thus, a functional inositol catabolic pathway and its proper regulation are important nutritional or signaling factors in the S. meliloti-alfalfa symbiosis.The sugar alcohol inositol, or cyclohexanehexol, occurs in several different stereoisomers, of which the myo-form (Fig. 1, compound 1) is the most abundant (1). myo-Inositol plays important structural and signaling roles in animal and plant cells (22). In the environment, myo-inositol mainly occurs in the phosphorylated form and is involved in the phosphate cycle of terrestrial and freshwater ecosystems (41). The stereoisomers D-chiro-and scylloinositol have recently attracted attention, because they have shown therapeutic potentials for diabetes and Alzheimer's disease, respectively (11, 21). Although there is only limited knowledge about the metabolism of D-chiro-and scyllo-inositol (25, 50), the catabolism of myo-inositol has been studied in a variety of microorganisms, including some members of the Firmicutes (17, 46, 51), Enterobacteriaceae (4,19,40), and Rhizobiaceae (16,29). The myo-inositol catabolic pathway and its regulation are best understood in the Gram-positive bacterium Bacillus subtilis. The B. subtilis iol genes are organized in a divergon comprising iolABCDEFGHIJ and iolRS (47-49). In the proposed inositol catabolic pathway, the myo-inositol dehydrogenase oxidizes myoinositol to its corresponding ketone 2-keto-myo-inositol (2KMI), which is then further catabolized by the actions of IolE, -D, -B, -C, -J, and -A (Fig. 1). The inducer of the inositol catabolic pathway in B. subtilis is the product of the IolC reaction, 2-deoxy-5-keto-D-gluconic acid 6-phosphate (DKGP; compound 6 in Fig. 1), which antagonizes the binding of the IolR repressor to the iol promoter region (51).Sinorhizobium meliloti, the nitrogen-fixing symbiont of alfalfa, can use myo-inositol as the sole carbon source (15). The idhA-encoded myo-inositol dehydrogenase had been s...

Rhizobial communities in symbiosis with legumes: genetic diversity, competition and interactions with host plants

“…It was evidenced that it might affect the structure of rhizobial populations [69]. Secondly, some data indicates as potential role of metabolic capabilities in rhizobial strain competitiveness including the ability to utilize specific carbon and energy sources, such as rhamnose, erythritol, mellibiose or other sugars, and homoserine or inositol [35,[71][72][73], as well as an overall metabolic potential of strains, i.e. the utilization of a variable set of substrates, including sugars, carboxylic acids and aminoacids [74,75].…”

The RpiR-Like Repressor IolR Regulates Inositol Catabolism in Sinorhizobium meliloti

“…Especially high concentrations have been found in legume plants (7,35). Sinorhizobium meliloti, the nitrogen-fixing symbiont of alfalfa, is able to catabolize myo-, scyllo-, and D-chiro-inositol (19). A functional inositol catabolic pathway and the regulatory gene iolR are required for successfully competing during host plant nodulation (19).…”

“…Sinorhizobium meliloti, the nitrogen-fixing symbiont of alfalfa, is able to catabolize myo-, scyllo-, and D-chiro-inositol (19). A functional inositol catabolic pathway and the regulatory gene iolR are required for successfully competing during host plant nodulation (19). Metabolism of inositol compounds has been linked not only to successful plant-bacterium interactions in members of the Rhizobiaceae but also to the survival of the animal pathogen Brucella inside its host (9,11,15,21).…”

“…1) in S. meliloti is carried out by the idhA-encoded myo-inositol dehydrogenase, yielding 2-ketomyo-inositol (2KMI; [IV] in Fig. 1) (12,19). Growth with scyllo-inositol ( [III] in Fig.…”

“…Growth with scyllo-inositol ( [III] in Fig. 1) as the sole carbon source requires the dehydrogenase encoded by the smc01163 gene (19). The deduced gene product of smc01163 displays an N-terminal signal peptide, suggesting a periplasmic location.…”