Location of Nitrogenase and Ammonia-assimilatory Enzymes in Bacteroids of Rhizobium leguminosarum and Rhizobium lupini

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Abstract
Summary: Cytoplasmic and membrane fractions were isolated from bacteroids of Rhizobium leguminosarum and R. lupini disrupted by lysozyme-induced lysis and sonication. The release of nitrogenase, glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 2.6.1.53) and glutamate dehydrogenase (EC 1.4.1.4) activities was compared with NADH oxidase and 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30) activities as markers for membrane and cytoplasmic enzyme activities respectively. Nitrogenase was localized in the cytoplasm. Most of the glutamate dehydrogenase activity was released on lysozyme treatment of intact bacteroids although some was released from membrane fractions on sonication, indicating a very superficial localization. Glutamate synthase was present at very low levels in R. leguminosarum bacteroids; the level in R. lupini bacteroids was higher and the enzyme was probably weakly membrane-bound. About 95% of the glutamine synthetase activity was in the nodule cytosol, and the supernatant of washed bacteroids had a high specific activity; the remaining activity was unevenly distributed between soluble and membrane fractions.

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