The Expression of a Third Nitrogenase in the Cyanobacterium Anabaena variabilis

Abstract

Physiological experiments indicate that Anabaena variabilis can express either a V- or a Fe-nitrogenase in addition to the conventional, M o-containing enzyme complex. The occurrence of the Fe-nitrogenase in A. variabilis can also be concluded from DNA - DNA hybridization experiments using cloned anfW or nifW probes coding for the smaller subunit (= nitrogenase reductase) of the Fe-nitrogenase from Azotobacter vinelandii or for the same subunit of the Mo-nitrogenase from Klebsiella pneumoniae. The cyanobacterium A. variabilis is the first phototroph found to contain all three nitrogenases wich has been described only for A. vinelandii as yet. Vanadium cannot substitute for Mo in nitrate reduction of A. variabilis.