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. Complementation of mutants to isolate nitrogen fixation genes using gene libraries. The net effect is a large amplification of gene expression.
Nodules are structures on plants where nitrogen fixation is done by symbiotic bacteria.
The plant provides the bacteria with fixed carbon produced by photosynthesis, while the bacteria supply fixed nitrogen to the plant. This means that even trace amounts of oxygen will be bound. In other words, LHb has a very high affinity for oxygen. The concentration of LHb in nodules is maintained at very high levels. This helps to ensure an anaerobic environment.