Description:
<jats:title>ABSTRACT</jats:title>
<jats:p>
<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">Rhodobacter capsulatus</jats:named-content>
fixes atmospheric dinitrogen via two nitrogenases, Mo- and Fe-nitrogenase, which operate under different conditions. Here, we describe the functions in nitrogen fixation and regulation of the
<jats:italic>rcc00574</jats:italic>
(
<jats:italic>cooA</jats:italic>
) and
<jats:italic>rcc00575</jats:italic>
(
<jats:italic>cowN</jats:italic>
) genes, which are located upstream of the structural genes of Mo-nitrogenase,
<jats:italic>nifHDK</jats:italic>
. Disruption of
<jats:italic>cooA</jats:italic>
or
<jats:italic>cowN</jats:italic>
specifically impaired Mo-nitrogenase-dependent growth at carbon monoxide (CO) concentrations still tolerated by the wild type. The
<jats:italic>cooA</jats:italic>
gene was shown to belong to the Mo-nitrogenase regulon, which is exclusively expressed when ammonium is limiting. Its expression was activated by NifA1 and NifA2, the transcriptional activators of
<jats:italic>nifHDK</jats:italic>
. AnfA, the transcriptional activator of Fe-nitrogenase genes, repressed
<jats:italic>cooA</jats:italic>
, thereby counteracting NifA activation. CooA activated
<jats:italic>cowN</jats:italic>
expression in response to increasing CO concentrations. Base substitutions in the presumed CooA binding site located upstream of the
<jats:italic>cowN</jats:italic>
transcription start site abolished
<jats:italic>cowN</jats:italic>
expression, indicating that
<jats:italic>cowN</jats:italic>
regulation by CooA is direct. In conclusion, a transcription factor-based network controls
<jats:italic>cowN</jats:italic>
expression to protect Mo-nitrogenase (but not Fe-nitrogenase) under appropriate conditions.
</jats:p>