POSTTRANSLATIONAL REGULATION OF NITROGENASE ACTIVITY BY ANAEROBIOSIS AND AMMONIUM IN AZOSPIRILLUM-BRASILENSE

In the microaerophilic diazotroph Azospirillum brasilense, the addition of fixed nitrogen or a shift to anaerobic conditions leads to a rapid loss of nitrogenase activity due to ADP-ribosylation of dinitrogenase reductase. The product of draT (DRAT) is shown to be necessary for this modification, and the product of draG (DRAG) is shown to be necessary for the removal of the modification upon removal of the stimulus. DRAG and DRAT are themselves subject to posttranslational regulation, and this' report identifies features of that regulation. We demonstrate that the activation of DRAT in response to an anaerobic shift is transient but that the duration of DRAT activation in response to added NH4+ varies with the NH4+ concentration. In contrast, DRAG appears to be continuously active under conditions favoring nitrogen fixation. Thus, the activities of DRAG and DRAT are not always coordinately regulated. Finally, our experiments suggest the existence of a temporary period of futile cycling during which DRAT and DRAG are simultaneously adding and removing ADP-ribose from dinitrogenase reductase, immediately following the addition of a negative stimulus.