Determination of C/N Ratios Required for De-Repression of Nitrogenase in Rhodobacter capsulatus

Phototrophic continuous and batch cultures of Rhodobacter capsulatus were employed to identify the C/N ratio above which nitrogenase is de-repressed. The cultures were grown with limiting am ounts of am m onium as source of bound nitrogen and with L-lactate or lm alate as sources of carbon and reducing equivalents. De-repression of nitrogenase was determ ined on the basis of the occurrence of dinitrogen fixation, acetylene reduction and n ifH p rom oter activities as well as on the basis of hydrogen evolution and nitrogenase poly­ peptides. In continuous culture, cells started to fix dinitrogen, to reduce acetylene, to activate the n ifH p rom oter and to form nitrogenase polypeptides, when consuming lactate per am m o­ nium at a C/N ratio of about 6 (this ratio represents the num ber of C and N atom s con­ sum ed). With m alate as carbon source all of the activities became detectable above a C/N ratio of about 8. Essentially the same C/N ratios were determ ined with batch cultures for the occurrence of N-lim itation of growth and hydrogen evolution. The experim entally d e te r­ mined C/N ratios for nitrogenase de-repression essentially agreed with C/N ratio of 5.8 and 7.8 calculated for the assimilation of amm onium and either lactate or m alate, into biomass of an elem ental com position of C H j.83N0.i830o.5This means that the occurrence of N-limitation and nitrogenase de-repression is defined by a threshold C/N ratio required for biomass production. As experim entally and theoretically shown, this ratio depends on the reduction state of the carbon source. It is concluded that the C/N ratio of nutrient consum ption rep re­ sents an intracellular signal which is directly translated into nitrogenase de-repression.