EFFECT OF LIGHT AND OXYGEN ON NITROGENASE ACTIVITY AND DINITROGENASE REDUCTASE (FE-PROTEIN) CONTENT IN AZOLLA-ANABAENA ASSOCIATION

Nitrogen fixation in Azolla is regulated by light, in vivo and in vitr o, and occurs only upon illumination. After long incubation in the dar k, illumination of Azolla caused a slow recovery of nitrogenase to max imal activity after 3 h without correlation with either photosynthesis or the content of soluble sugars. Cyanobiont heterocyst extracts immu nobloted with anti-Klebsiella or anti-Rhodospirillum Fe-protein antibo dies showed two reacting polypeptides, one of 30 and the other of 36 k Da. The content of the 30 kDa polypeptide was not affected by transiti on of Azolla to 18 h in the dark while the 36 kDa polypeptide complete ly disappeared in the dark and reappeared within 10 to 30 min of reill umination. The cyanobiont nitrogenase was active only in the presence of a detectable amount of the 36 kDa polypeptide, suggesting that it i s the active form of the Fe-protein. Illumination of Azolla with high light intensity after a short dark period caused inhibition of nitroge nase activity and reduced the content of the 36 kDa polypeptide. Nitro genase was inactivated by high O-2 only in the dark while in the light nitrogenase was not inhibited and the content of the 36 kDa Fe-protei n polypeptide was retained.