NITROGEN ASSIMILATION IN SOYBEAN NODULES : II. ^<15>N_2 Assimilation in Bacteroid and Cytosol Fractions of Soybean Nodules

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^<15>N assimilation was studied in bacteroid and cytosol fractions of soybean nodules.In the first experiment, after exposing the intact nodules to ^<15>N_2 for 5 min and 10 min, most of the fixed ^<15>N was detected in cytosol fraction.In cytosol fraction, ^<15>N content of glutamine was the highest and followed by glutamic acid, alanine, and allantoin in this sequence, whereas, in bacteroid fraction, glutamic acid showed the highest ^<15>N content and alanine and glutamine followed.In the second experiment, ^<15>N assimilation of various ^<15>N-labeled compounds in the separated bacteroid and cytosol fractions was investigated.In the separated bacteroid fraction which was fed with ^<15>NH_4,^<15>N was incorporated very rapidly into glutamic acid, alanine, and aspartic acid, but very slowly into glutamine. From these results, it was suggested that most of the fixed ammonia was exported to cytosol and assimilated via glutamine synthetase to glutamine, then via glutamate synthase to glutamic acid, and from these compounds various nitrogenous compounds were formed, but in bacteroids glutamate dehydrogenase and alanine dehydrogenase played an important role in the assimilation of fixed ammonia though quantitatively the contribution to ammonia assimilation in nodules was much less compared with cytosol.
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