ABSTRACT
The ex-planta nitrogenase efficiencies of six electron transport mutant strains of B. japonicum 61A76 were studied in defined liquid media under microaerobic conditions. The extent of cytochrome deficiency of the mutants was associated with the loss of the ability of these strains to develop acetylene reduction activities. Mutants which were totally deficient in cytochrome AA3 and C with reduced amount of cytochrome B, developed little or no nitrogenase activity. Other mutants which were deficient in cytochrome AA3 or which had reduced levels of all cytochromes, had a reduced or delayed nitrogenase as compared with the wild type strain. From the spectral characteristics, it seems that under oxygen limitation, terminal cytochrome oxidase almost resembled those of the symbiotic bacteria. Growth efficiencies were correlated with the cytochrome content of the bacteria