Changes in the regulatory form of Rhodospirillum rubrum nitrogenase as influenced by nutritional and environmental factors.

The photosynthetic bacterium Rhodospirillum rubrum regulates the activity of its nitrogenase (N2ase) by interconverting the enzyme into three distinct enzymatic species: N2ase A (a fully active form) and two regulatory forms, N2ase Ractive and N2ase Rinactive. N2ase R is distinguished from N2ase A in vitro by the requirement of its Fe protein for activation by a Mn2+-dependent activating factor. N2ase is converted from the A to the R form in response to certain environmental factors such as carbon starvation, depletion of intracellular adenosine triphosphate, or the addition of NH4+ (or glutamate) to a culture of N-starved cells. The rapid inhibition of R. rubrum N2ase in vivo by NH4+ was shown to result from the conversion of N2ase A to N2ase Rinactive. On depletion of NH4+ from the culture, whole-cell N2ase activity returned; however, the enzyme remained in the R form. Unlike the effect of NH4+, adding glutamate to cells containing N2ase A did not inhibit in vivo activity, but converted the enzyme to the R form (N2ase Ractive). Although glutamate-induced N2ase R formation was much slower than the NH4+-induced reaction, it occurred in the presence of rifampin, indicating that de novo protein synthesis was not involved. This suggested that N2ase R was formed by a modification of N2ase A. Although glutamine synthetase in involved in the conversion of N2ase A to R, the adenylylation state of glutamine synthetase appears not to be involved in regulating this nitrogenase reaction.