A functional myo-inositol catabolism pathway is essential for rhizopine utilization by Sinorhizobium meliloti

Abstract

Rhizopine (~-3-O-methyl-scy//o-inosamine) is a symbiosis-specif ic compound found in alfalfa nodules induced by specific Sinorhilobium meliloti strains. It has been postulated that rhizobial strains able to synthesize and catabolize rhizopine gain a competitive advantage in the rhizosphere. The pathway of rhizopine degradation is analysed here. Since rhizopine is an inositol derivative, it was tested whether inositol catabolism is involved in rhizopine utilization. A genetic locus required for the catabolism of inositol as sole carbon source was cloned from S. meliloti. This locus was delimited by transposon Tn5 mutagenesis and its DNA sequence was determined. Based on DNA similarity studies and enzyme assays, this genetic region was shown to encode an S, meliloti myo-inositol dehydrogenase. Strains that harboured a mutation in the myo-inositol dehydrogenase gene (idhA) did not display myoinositol dehydrogenase activity, were unable to utilize myo-inositol as sole carbodenergy source, and were unable to catabolize rhizopine. Thus, myoinositol dehydrogenase activity is essential for rhizopine utilization in 5. meliloti. I I

The complete sequence of the 1,683-kb pSymB megaplasmid from the N 2 -fixing endosymbiont Sinorhizobium meliloti

“…The histidine utilization genes (hutUGHIL), whose products are responsible for the conversion of L-histidine to glutamate and formamide, are present as a single operon. Previously identified genes involved in the utilization of poly3-hydroxybutyrate, the ␣-galactosides, melibose, raffinose (apaA, agpL, agpT), and lactose (lacEF-GZK), including the two previously hypothesized lacZ genes, were identified (21,(52)(53)(54)(55).…”

The fifth gene of the iol operon of Bacillus subtilis, iolE, encodes 2-keto-myo-inositol dehydratase

“…Several micro-organisms, including Bacillus subtilis (Yoshida et al, 1997), Cryptococcus melibiosum (Vidal-Leiria & van Uden, 1973), Aerobacter aerogenes (reclassified as Enterobacter aerogenes/Klebsiella mobilis) (Berman & Magasanik, 1966a), Rhizobium leguminosarum bv. viciae (Poole et al, 1994), Sinorhizobium meliloti (Galbraith et al, 1998) and Sinorhizobium fredii (Jiang et al, 2001), can grow on inositol as the carbon source. It was thought that bacterial inositol catabolism is only required for efficient utilization of this compound.…”

Identification of a Functional 2-keto-myo-Inositol Dehydratase Gene ofSinorhizobium frediiUSDA191 Required formyo-Inositol Utilization

“…7) On the other hand, in S. meliloti, the utilization of rhizopine (L-3-O-methyl-scyllo-inosamine), a symbiosis-specific compound found in alfalfa nodules, has been linked to myo-inositol catabolism. 6,15) These observations suggest that bacterial myo-inositol catabolism, particularly in S. fredii USDA191, might play an important role in Rhizobium-legume symbiosis.…”

“…1,27) It has been proposed that myo-inositol catabolism is linked to rhizopine utilization. 6) Mutants of R. leguminosarum bv. trifolii compromised in rhamnose catabolism exhibit a competitive disadvantage in clover nodulation, 29) but mutants affecting the catabolism of sorbitol and adonitol vie successfully with the wild-type strain.…”

“…Several microorganisms, including Bacillus subtilis, 1) Cryptococcus melibiosum, 2) Aerobacter aerogenes (reclassified as Enterobacter aerogenes/Klebsiella mobilis), 3,4) Rhizobium leguminosarum bv. Viciae, 5) Sinorhizobium meliloti, 6) and S. fredii 7) utilize myo-inositol as a carbon source.…”