Genetics of Nitrogen Fixation in Free-Living Organisms

Using the acetylene-reduction assay, a long list of organisms has been prepared that are able to fix nitrogen under free-living conditions (Table 1). This list contains anaerobic, microaerobic, facultative and aerobic microorganisms. It is worth noting that all these microorganisms are of procaryotic origin and that no eucaryotic organism with the capacity to fix nitrogen is known. Although a lot is known about the physiology and biochemistry of free-living, nitrogen-fixing organisms, only very few have been used for genetic studies of the nitrogen-fixation process. The facultative microorganism Klebsiella pneumoniae has been mainly used to elucidate the genetic background of biological nitrogen fixation and is, therefore, extensively discussed in this paper. Besides Klebsiella, the genetics of free-living, nitrogen-fixing bacteria is still very virginal. For Azotobacter vinelandii, nitrogenase-defective mutants have been reported.1-3 Using the Azotobacter transformation system, Bishop and Brill4 were able to locate the mutations as a cluster on the chromosome. Mutants defective in nitrogen fixation (nif -) were also isolated for cyanobacteria,5-7 but, due to the lack of a gene-transfer system, no further genetic studies could be performed with these mutants.