Regulatory effect of ammonium on the nitrogenase activity of Rhodobacter sphaeroides and Rhodobacter capsulatus is not mediated by ADP-ribosylation of the Fe-protein of nitrogenase

During the cultivation of Rhodobacter sphaeroides in an ammonium-limited ch emostat, the effect of nitrogenase switch-off was observed at a wide range of dilution rates (from 0.04 to 0.18 h(-1)), i.e., at various levels of nit rogen limitation. ADP-ribosylated nitrogenase (ribosylation is catalyzed by DRAT enzyme) was not revealed in cells. The results of hybridization of th e total DNA of Rba. sphaeroides with the draT and draG genes of Rhodospiril lum rubrum and Rhodobacter capsulatus suggest that these genes are absent i n Rba. sphaeroides. The effect of nitrogenase switch-off by ammonium was al so observed for DRAT/DRAG defective mutants of Rba, capsulatus (W107I and W 107II) grown in batch culture. These findings testify to the functioning in Rba. sphaeroides and Rba. capsulatus cells of a mechanism of nitrogenase s witch-off by ammonium that is alternative to ADP-ribosylation of the Fe-pro tein subunit of nitrogenase, Evidence was obtained against the suggestion t hat nitrogenase is regulated by ammonium via changes in the degree of reduc tion of the cellular pool of NAD.