On the basis of enzyme assays, -inositol appears to be catabolized via 2-keto--inositol and D-2,3-diketo-4-deoxy -inositol in bv. , as occurs in aerogenes. The first two enzymes of the pathway, -inositol dehydrogenase and 2-keto--inositol dehydratase were increased 7- and 77-fold, respectively, after growth of on -inositol compared to glucose. Cells of grown on glucose as the carbon source and then resuspended in -inositol, increased -inositol-dependent O consumption by sixfold in 3 h, confirming this to be an inducible pathway. Succinate, malate and glucose exhibited strong catabolite repression of the -inositol catabolic pathway with -inositol dehydrogenase and 2-keto--inositol dehydratase being repressed by at least 68% and 80%, respectively, in all cases. A dicarboxylate transport mutant () was unable to repress either enzyme when grown on -inositol and succinate. There was no repression of the first two enzymes of the -inositol catabolic pathway in the background of constitutive expression of the dicarboxylate transport system, indicating a dicarboxylate must be present to cause repression. The data imply that dicarboxylates are required intra-cellularly to repress these enzymes. Three transposon mutants were isolated that cannot grow on -inositol as the sole carbon source and were unable to induce either -inositol dehydrogenase or 2-keto--inositol dehydratase. The mutants were unaffected in the ability to nodulate peas and vetch. Furthermore, vetch plants infected with one mutant (RU360) reduced acetylene at the same rate as plants infected with the wild type. Bacteroids did not oxidize -inositol, nor were the catabolic enzymes detected, confirming -inositol is not important as an energy source in bacteroids. The possible role of -inositol in the rhizosphere is considered.


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