As a result, nitrogen fixation cannot be done in the presence of oxygen. Despite the extreme oxygen-sensitivity of nitrogenase, some microbes that live in aerobic environments can fix nitrogen. Azotobacter has an unusually high rate of respiration. It consumes oxygen so fast that the intracellular concentration of oxygen is kept low enough that nitrogenase is not inhibited. The facultative aerobe Klebsiella fixes nitrogen only when it is in an oxygen-free environment.
Filamentous cyanobacteria have specialized cells called heterocysts that fix nitrogen.
Diffusion of oxygen into these cells is limited. The product of nifA is a transcriptional activator protein that in turn activates expression of many nif and fix genes. An important point is that this regulatory cascade that results in the expression of genes involved in nitrogen fixation will not occur in the presence of oxygen. This makes sense - since nitrogenase is not functional under oxic conditions, there is no point in expressing these genes when oxygen is present. Transposon insertion mutagenesis.