Genetics of plant-microbe nitrogen-fixing symbiosis

Synopsis A wide variety of plant-microbe nitrogen-fixing symbioses which include cyanobacteria as the nitrogenfixing partner exist. While some information has been gathered on the biochemical changes in the cyanobacterium upon entering into symbiosis, very little is known about the accompanying changes at the genetic level. Much of our present knowledge of the organisation and control of expression of nitrogenfixation (nif) genes is derived from studies of the free-living diazotrophKlebsiella pneumoniae. This organism thus provides a model system and source of experimental material for the genetic analysis of symbiotic nitrogen fixation. We describe the use of clonedK. pneumoniaegenes for nitrogen fixation and its regulation in the genetic analysis' of nitrogen fixation in cyanobacteria which can enter into symbiosis with plants. These studies reveal some dissimilarities in the organisation ofnifgenes and raise questions as to the genetic control of nitrogen fixation in symbiosis.