Effect of Denitrification on Nitrogen Fixation in Light in a Photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

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Abstract
The effect of denitrification on N2 fixation was studied in a denitrifying photosynthetic bacterium, Rhodopseudomonas sphaeroides forma sp. denitrificans. KNO3 inhibited diazotrophic growth in light, but N03−-dependent diazotrophic growth in darkness was found. N03− inhibited C2H2 reduction activity in light in cells grown with NO3−. NO3−-dependent C2H2 reduction activity in darkness also was present in cells grown with N2 plus NO3−, but not in cells grown on glutamate with NO3−. NO3− repressed the synthesis of nitrogenase in light. This repression was not overcome by the addition of methionine sulfoximine. The inhibitory and repressive effect of NO3− was not mediated by the N02− produced from N03− nor by the NH4+ excreted into the medium. But, as N2 fixation is controlled by O2 (redox control) it seems to be mediated through the denitrification processes. Much of the glutamine synthetase was adenylylated in cells grown with N03− and its adenylylation state closely parallelled nitrogenase activity in the cells.