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

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RESUMO

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.

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