The transport and catabolism of C4-dicarboxylic acids have been further studied in Rhizobium leguminosarum MNF3841. Uptake of [14C]succinate was induced in free-living cells of strain MNF3841 within 12 min of exposure to succinate and reached a maximum rate within 60 min. Free-living cells of strain MNF3841 oxidizing fumarate accumulated pyruvate when treated with arsenite, an inhibitor of pyruvate dehydrogenase. Generation of pyruvate from C4-dicarboxylates was accomplished by malic enzyme. Although malic enzyme was present in free-living cells grown on sucrose, higher activities were observed when fumarate or l-arabinose was the growth substrate. In crude extracts, malic enzyme activity required either NAD+ or NADP+ as cofactor, together with Mn2+, and was stimulated by K+.
Bacteroids from pea nodules transported [14C]succinate immediately after isolation, and contained twofold higher activities of both malic enzyme and pyruvate dehydrogenase than those found in fumarate-grown free-living cells. These data indicate that C4-dicarboxylates are available to nodule bacteroids and that they are catabolized through the tricarboxylic acid cycle using malic enzyme and pyruvate dehydrogenase to generate acetyl-CoA.
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