SUMMARY: Excised soybean root nodules exposed to an atmosphere containing an excess of N incorporated the label first into the centrifugal fraction containing the intracellular membrane envelopes and small amounts of bacteroid cell walls. The N concentration then increased in the soluble portion of the nodules. The bacteroid fraction was not labelled after 2 hr. exposure of the nodules to N. In ageing nodules incorporation of N into the soluble fraction declined before incorporation into the membrane fraction. The inhibitory effects of CO, NO and H on N incorporation in the various fractions were studied; the results suggested differential inhibition. The membrane fraction contained 8.5% (w/v) total N, 37% (w/w) lipid, had a negligible O uptake in the presence of substrates and had an absorption spectrum suggestive of the presence of porphyrin compounds. Further fractionation of the membrane fraction indicated that the N was associated with the lighter particles and was only partially soluble in 3N-HC1. The possibility that the membrane fraction contained the site of primary N activation is discussed.


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