Molecular genetics of symbiotic nitrogen fixation.

Although most organisms, including all eucaryotes, cannot assimilate atmospheric dinitrogen (N2), a group of evolutionarily divergent procaryotes are able to reduce N2 directly to ammonia (nitrogen fixation) under a variety of physiological conditions. The molecular genetics of nitrogen fixation has been studied primarily in Klebsiella pneumoniae because of its close taxonomic relation to Escherichia coli. K. pneumoniae contains a cluster of 17 contiguous nifgenes, which are all transcribed in the same direction and which are divided into seven or eight operons. Three of these genes (nifK, nifD and nifH) encode nitrogenase, an enzyme complex composed of two characteristic components: a molybdenum-iron-containing protein, which reduces substrate, and an iron-containing protein, which transfers electrons to the molybdenum-iron-containing protein. The remaining nif gene products either synthesize an nif-specific ironmolybdenum cofactor or carry out regulatory, electron transport, protein maturation or unknown functions. The physical location of each nif gene has been mapped to within 100 bp with transposable genetic elements as mutagens (Riedel et al., PNAS 76, 28662870, 1979) and the regulatory circuitry controlling nif gene expression in K. pneumoniae has been deciphered (for a recent review, see Roberts and Brill, Ann. Rev. Microbial. 35, 207-235, 1981). In contrast with K. pneumoniae, which fixes nitrogen in a free-living form, bacteria of the genus Rhizobium normally fix nitrogen only in symbiotic association with plants of the family Leguminoseae. Symbiotic nitrogen fixation occurs in root nodules-complex, highly differentiated structures formed by the interaction of bacteria and plant. Rhizobia usually gain entry into the legume root by penetrating root hairs. In response to bacterial invasion, the plant produces a cellulose tube called an infection thread, which surrounds the bacteria, penetrates root cells and ramifies. The bacteria within the thread proliferate, and cells in the inner root cortex, which are normally terminally differentiated, begin to divide, forming a meristematic region that produces a nodule ranging in size from 1 to 10 mm in different legume species. Finally, various branches of the infection thread release their bacteria intracellularly, and the bacteria differentiate into a new form called a bacteroid and derepress nitrogen-fixation genes. The host cells also show changes in protein composition (Legocki and Verma, Cell 20, 153Minireviews