Regulation of nitrate reductase in cyanobacteria. Repression-derepression control of nitrate reductase apoprotein in the cyanobacterium Nostoc muscorum

The repression-derepression control of Nostoc muscorum nitrate reductase was studied with regard to the Mo-cofactor and apoprotein levels. It was found that the synthesis of Mo-cofactor is constitutive but the apoprotein is subject to the repression-derepression control. In NH₄⁺ medium apoprotein synthesis was repressed and in N₂ and NO₃^? media apoprotein synthesis was derepressed. The apoprotein levels were similar in NO₃^? and N₂ media; however, the nitrate reductase activity was lower in N₂ medium due to lower Mo-cofactor activity. The lower Mo-cofactor activity in N₂-fixing conditions as compared to that in non-N₂-fixing conditions was consistent with the earlier view that the Mo-cofactor of nitrate reductase may be a precursor for FeMo-cofactor of nitrogenase.