Regulation of nitrogenase in the photosynthetic bacterium, Rhodobacter capsulatus

摘要

Previous studies have shown that the regulation of nitrogenase synthesis and activity in Rhodobacter capsulatus is complex (Masepohl, Klipp 1996). Transcription of Fe-protein requires active NifA protein and R, capsulatus contains two copies of NifA, NifA1 and NifA2, which can function in this regard. Transcription of NifA requires the R. capsulatus NtrC and NtrB homologs whose activity is modulated in response to the nitrogen status of the cell. Once synthesized, the activity of NifA is controlledby fixed nitrogen in a manner that has been unclear. After its synthesis, nitrogenase is subject to several controls. Reversible ADP-ribosylation of Fe-protein, carried out by DraT and DraG, has the potential for modulating nitrogenase activity in response to the addition of NH_3 or sudden changes in light intensity. As well, there appears to be an additional mechanism that is capable of regulating nitrogenase activity in R. capsulatus, a control over nitrogenase activity that is independent of the ADP-ribosylation system since it can be observed in strains that lack DraT, DraG. These systems for the regulation of nitrogenase activity are in turn sensitive to the cellular nitrogen status. Not only are these responses seen in response to sudden changesin environmental conditions, e.g. addition of NHs, light deprivation, but they are also observed with cells that are under steady state conditions. Thus cells that are actively fixing N_2, cells growing on glutamate, or cells grown on moderate amounts of limiting NH_3 all show some degree of ADP-ribosylation of Fe-protein (Yakunin et al. 1999). The degree of modification observed has been shown to be correlated with the intracellular pool of fixed nitrogen.