SUMMARY: Bacteroids isolated from stem nodules of were able to reduce CH when glucose or succinate was the energy-yielding substrate. To be efficient, glucose always required low O concentrations, both with free O and with O delivered by leghaemoglobin extracted from French-bean nodules. Lactate detected in the stem nodule cytosol supported CH reduction by bacteroid preparations, but the O tensions required for optimal activity were always higher than those defined for glucose, as predicted by the model of Bergersen&Trinchant (J. theor. Biol. 115, 93-102, 1985). In contrast, lactate was only present at very low concentrations in the cytosol of French-bean nodules and was unable to support CH reduction by bacteroids isolated from these nodules. Lactate dehydrogenase (EC extracted from stem nodule bacteroids and free-living cells of sp. () exhibited a higher affinity for lactate than did the enzyme from biovar or and their symbiotic forms. Lactate appears to be a specific energy-yielding substrate supporting N fixation by Sesbania bacteroids and could play a major role when fermentative processes take place in the nodules.


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