Nitrogen fixation by the unicellular cyanobacterium Gloeothece. Nitrogenase synthesis is only transiently repressed by oxygen

The extent of recovery of nitrogenase activity of Gloeothece transferred from an atmosphere of O2 to air depended on the duration of exposure to O2. Activity recovered at increasing rates after up to 24 h exposure to O2 and a lag before detection of activity, present after short (1 h) exposure times, disappeared with longer exposures. Synthesis of nitrogenase de novo was implicated, since chloramphenicol, tetracycline, or repressive levels of NH+4, prevented recovery of activity. Specific radioimmunoassay of the rate of synthesis of the MoFe protein of nitrogenase under O2 correlated well with the activity measurements, and indicate that a shift from air to O2 only transiently represses nitrogenase synthesis.