Nitrogenase reductase: A functiona Rhizobium phaseoli (DNA reiterations/identical genes/symbiotic nitrogen fixation/nucleoi

The complete coding sequence of the nitro- genase reductase gene (nifH) is present in three different re- gions of a Rhizobium phaseoli symbiotic plasmid. Homology between two of the regions containing nifH coding sequences extends over 5 kilobases. These in turn share 1.3 kilobases of homology with the third region. The nucleotide sequences of the three nitrogenase reductase genes were found to be identi- cal. Site-directed insertion mutagenesis indicated that none of the three genes is indispensable for nitrogen fixation during symbiosis with Phaseolus vulgaris. This implies that at least two of the reiterated genes can be functionally expressed. Some prokaryotic organisms are able to fix atmospheric ni- trogen due to the presence of the nitrogenase enzyme com- plex. This complex is composed of two enzymes: nitrogen- ase (component I) and nitrogenase reductase (component II). Component I is composed of two c subunits and two 3 subunits encoded by the nifD and nifK genes, respectively. Component II is composed of two identical subunits en- coded by the nifH gene. The genes coding for the nitrogen- ase polypeptides were originally characterized in Klebsiella pneumoniae (1-3). In this organism nif genes H, D, and K are organized in one transcriptional unit. Nitrogenase struc- tural genes are highly conserved in evolution. Gene probes isolated from K. pneumoniae cross-hybridize with equiva- lent sequences from a wide variety of nitrogen-fixing orga- nisms (4). This has facilitated the analysis of these genes in other organisms, including bacteria belonging to the genus Rhizobium, which fix nitrogen in symbiosis with the roots of legumes (5-10). For Rhizobium phaseoli, the symbiont of Phaseolus vul- garis, we reported the reiteration of nitrogen-fixation gene sequences (11). In this paper we present a structural and functional analysis of reiterated nitrogenase reductase DNA sequences. We conclude that nitrogenase reductase is a functional multigene family in R. phaseoli.