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

In the microaerophilic diazotroph Azospirillum brasilense, the additio n of fixed nitrogen or a shift to anaerobic conditions leads to a rapi d loss of nitrogenase activity due to ADP-ribosylation of dinitrogenas e reductase. The product of draT (DRAT) is shown to be necessary for t his modification, and the product of draG (DRAG) is shown to be necess ary 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 demonstrat e that the activation of DRAT in response to an anaerobic shift is tra nsient but that the duration of DRAT activation in response to added N H4+ varies with the NH4+ concentration. In contrast, DRAG appears to b e continuously active under conditions favoring nitrogen fixation. Thu s, the activities of DRAG and DRAT are not always coordinately regulat ed. Finally, our experiments suggest the existence of a temporary peri od of futile cycling during which DRAT and DRAG are simultaneously add ing and removing ADP-ribose from dinitrogenase reductase, immediately following the addition of a negative stimulus.