SUMMARY: (CC711) was used to infect soybean seeds from which plants were grown. From the root nodules, bacteroid suspensions with initial rates of nitrogen fixation as high as those calculated for bacteroids in intact nodules were prepared. Oxygen, which was required for fixation by intact nodules and bacteroid suspensions, caused the eventual loss of bacteroid nitrogen-fixing ability, accompanied by an increase in O-uptake. In intact nodules and in bacteroid suspensions, increasing O pressures resulted in higher values for of nitrogen fixation. also increased with increasing O and this was shown to be consistent with the characteristics of nitrogen fixation by anaerobic cell-free extracts of bacteroids which required an energy source (ATP), and a reductant (sodium dithionite).

Higher concentrations of carbon monoxide were required to inhibit nitrogen fixation by intact nodules than by bacteroid suspensions. Carbon monoxide was a competitive inhibitor of nitrogen fixation in bacteroid suspensions and (CO) and (N) values showed that the nitrogenase had about 30 times the apparent affinity for CO that it had for N.

In cell-free extracts of bacteroids, the nitrogen-fixing activity remained in the supernatant fluid after centrifugation at 100,000 for 30 min. The extracts were inactivated at 0°. The for nitrogen fixation by cell-free extracts was variable, 62-118 (N concentration in mm. Hg. pressure), compared with 50-60 for intact nodules and 20 for intact bacteroids when for these was measured in the range of O in which it was only slightly affected by O. The for inhibition of nitrogen fixation by CO in extracts was similar to values obtained with intact bacteroids.


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