Control by light of whole-cell nitrogenase activity and of nitrogenase andbacteriochlorophyll a formation in Rhodobacter capsulatus strain B10S

Control of nitrogenase and bacteriochlorophyll a (BChl) by light was studie d under steady-state conditions with continuous cultures of Rhodobacter cap sulatus BIOS supplied with malate and growth-limiting amounts of ammonium. Consumption of malate and, correspondingly, the C/N ratio at which malate a nd ammonium were consumed increased when illumination was increased from 3 to approximately 30 klx and became constant at higher illuminations of up t o 30 klx. Essentially the same kinetics were observed with respect to nitro genase activity of cells, contents of nitrogenase polypeptides, and nifH pr omoter activity. Substrate consumption was half-maximal at 8 klx and was in dependent of the presence of nitrogenase. Therefore, it is concluded that l ight controls the C/N ratio (a quantitative measure of the nitrogen status of cells), which in turn is involved in the control of nitrogenase at the l evel of nif promoter activity. Post-translational regulation of nitrogenase activity by ADP-ribosylation was not observed under steady-state condition s, but it took place when illumination was suddenly decreased to the range where malate consumption and, consequently, the C/N ratio decreased. Irresp ective of the presence or absence of nitrogenase, specific BChl contents of the cultures were constant above 20 klx, and they increased at lower illum inations. These results do not confirm a recently proposed link between nit rogen fixation and photosynthesis as represented by BChl.