Analysis of Cyanothece sp. BH68K mutants defective in aerobic nitrogen fixation
1995
Reimels, A.J. | Reddy, K.J.
The unicellular cyanobacterium, Cyanothece sp. BH68K, is capable of performing both oxygen-sensitive nitrogen fixation and oxygenic photosynthesis within a single cell. To understand the oxygen protection mechanisms of nitrogenase, mutants defective in nitrogen fixation (Nif-) were isolated by use of diethyl sulfate as a mutagen. Out of 24 mutants screened, 6 mutants could not express nitrogenase activity under aerobic conditions, but expressed activity under anaerobic conditions (Fox-); 4 mutants showed no activity under both aerobic and anaerobic conditions (Fix-); and the remaining mutants were impaired in both aerobic and anaerobic nitrogenase activity (Imp). Respiratory oxygen consumption and photosynthetic oxygen evolution were analyzed in the wild-type and in two Fox- mutants. In the wild-type, the appearance of high aerobic nitrogenase activity was correlated with an increase in dark respiration, whereas no such increase was seen in the Fox- mutants. We propose that in Fox- mutants, respiratory oxygen consumption plays an important role in maintaining aerobic nitrogenase activity.
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