The onset and cessation of the synthesis of denitrification enzymes
of Pseudomonas stutzeri were investigated by using continuous
culture
and defined dissolved oxygen levels covering the full range of
transition
from air saturation to complete anaerobiosis. Expression of nitrate
reductase,
nitrite reductase (cytochrome cd1), and N2O
reductase was controlled by discrete oxygen levels and by the nature of
the nitrogenous oxide available for respiration. N2O
reductase was synthesized constitutively at a low level; for enhanced
expression,
oxygen concentrations were required to decrease below 5 mg of O2
per liter. The threshold values for synthesis of nitrate reductase and
cytochrome cd1 in the presence of nitrate were ca. 5 and ca.
2.5
mg of O2 per liter, respectively. With nitrous
oxide
as the respiratory substrate, nitrite reductase was again the most
sensitive
to oxygen concentration; however, thresholds for all denitrification
enzymes
shifted to lower oxygen levels. Whereas the presence of nitrate
resulted
in maximum expression and nearly uniform induction of all reductases,
nitrite
and nitrous oxide stimulated preferably the respective enzyme
catalyzing
reduction. In the absence of a nitrogenous oxide, anaerobiosis did not
induce enzyme synthesis to any significant degree. The accumulation of
nitrite seen during both the aerobic-anaerobic and anaerobic-aerobic
transition
phases was caused by the differences in onset or cessation of synthesis
of nitrate and nitrite reductases and an inhibitory effect of nitrate
on
nitrite reduction.