Effects of inorganic nitrogen compounds on the activity and synthesis of nitrogenase in Gloeothece (Nageli) sp. ATCC 27152

Addition of 2 mM nitrite or ammonium to aerobically incubated cultures of G loeothece rapidly inhibited N-2 fixation (measured as acetylene reduction). In contrast, 2 mM nitrate inhibited N-2 fixation less rapidly and less ext ensively, and often temporarily stimulated nitrogenase activity. The inhibi tory effects of both nitrate and ammonium could be prevented by addition of 3 mM L-methionine-DL-sulphoximine, suggesting that the true inhibitor of N -2 fixation was an assimilatory product of ammonium rather than either ammo nium or nitrate itself. The inhibition of N-2 fixation by nitrite could not , however, be prevented by addition of L-methionine-DL-sulphoximine. On the other hand, nitrite (unlike nitrate and ammonium) did not inhibit N-2 fixa tion in cultures incubated under a gas phase lacking oxygen. These findings suggest that the mechanism whereby nitrite inhibits N-2 fixation in Gloeot hece differs from that of either nitrate or ammonium. The inhibitory effect of nitrite on N, fixation did not involve reduction of nitrite to nitric o xide, though nitric oxide was a potent inhibitor of nitrogenase activity in Gloeothece. Nitrate and nitrite inhibited the synthesis of nitrogenase in Gloeothece, while ammonium not only inhibited nitrogenase synthesis but als o stimulated degradation of the enzyme. In addition, all three compounds fa voured the appearance of the Fe-protein of nitrogenase in its larger, presu med inactive, form.