Rhizobium Meliloti Symbiotic Nitrogen Fixation: Identification of NIF and FIX Genes; The NIF FIX Cascade Regulatory Pathway

Abstract

Rhizobium meliloti fixes nitrogen in symbiotic association with a limited number of legumes among which alfalfa (Medicago sativa). During the past 15 years it has been the subject of intense genetic studies with the result that it is now one of the best-known nitrogen-fixing organisms. It has largely benefitted from the recent progress in bacterial genetics and molecular biology and conversely some of the most significant technological advances in bacterial genetics have been achieved using R. meliloti (or other Rhizobium species) as a model system. Devising new tools for genetic studies of symbiotic nitrogen fixation was necessary because mutant phenotypes cannot be observed easily. Most Rhizobium species fix nitrogen only in the symbiotic state. Nitrogen fixation takes place in a specialized plant organ, the nodule which constitutes an adapted ecological niche for the nitrogen-fixing differentiated bacteroids (see the review by Long 1989). Therefore the screening of bacterial mutants deficient for nitrogen fixation needs time-consuming plant inoculation. Transposon mutagenesis was a first significant advance because it allows a positive screening of the mutagenic event (Beringer et al. 1978). Reverse genetics allows to identify the phenotype encoded by a given DNA sequence, for example one which shows homology with an already identified gene of another organism (Ruvkun and Ausubel 1981).