Advances in the Genetics of Free-Living and Symbiotic Nitrogen Fixing Bacteria

It is evident that the knowledge about biological nitrogen fixation was enormously increased when genetic systems in free-living and symbiotic nitrogen fixing bacteria were developed and used for the analysis of their nif (nitrogen fixation) genes. Due to its close relationship to Eschericia coli, Klebsiella pneumoniae, a facultatively anaerobic procaryotic microorganism, was initially selected to analyze nif genes (Streicher et al., 1971; Dixon, Postgate, 1971). For other Gram-negative nitrogen fixing microorganisms, genetic systems were recently developed and are now used to compile information in the field of nif genetics. To review all the information available would certainly exceed the scope of this paper. Therefore, we concentrate on three different nitrogen fixing species which are currently being analyzed in our laboratory: the facultative anaerobe Klebsiella pneumoniae, the phototrophic Rhodopseudomonas capsulata and the symbiotic nitrogen fixing Rhizobium meliloti. For these three species, the genetic techniques developed will be outlined. It is our special intention to show that break-throughs are often achieved following the development of new methodology.