diazotrophicus grows well in air when a source of
fixed-nitrogen is present but most fixed-nitrogen sources
must be removed and the O2 concentration over the culture
lowered for significant derepression of nitrogenase activity
[6]. Using glucose as an efficient carbon source with little or
no fixed-nitrogen present, we found reasonably good
nitrogenase activity when G. diazotrophicus cells were
grown with 5% O2 in the atmosphere over the culture.
However, the relatively small amounts of Gd2 recovered
from our preparations suggest that we did not achieve
optimal conditions. The large difference in the amount of
Gd1 present compared to Gd2 is most likely not due to any
differential expression of the structural genes, all of which
reside in the same operon [22], but rather to the organism’s
inability to protect its nitrogenase components from
subsequent damage, possibly by O2. The yield of G.
diazotrophicus nitrogenase, especially Gd2, might be
improved either by changing the O2 concentration used
when derepressing the nif genes or by adjusting the time at
which the O2 step-down is initiated after the start of
derepression. Alternatively, yield may be improved if the
cells were adapted to microaerobic conditions before
exposure to fixed-nitrogen limitation. However, under our