Reiteration of nitrogen fixation gene sequences in Rhizobium phaseoli

Bacteria of the genus Rhizobium form symbiotic nitrogen-fixing relationships with legumes. To investigate the organization of the nitrogen-fixation ( nif ) genes in Rhizobium phaseoli , which nodulates Phaseolus vulgaris , we have taken advantage here of the sequence homology between the nif D and nif H genes of Klebsiella pneumoniae and those of other nitrogen-fixing species 1–3 . Strains from different geographical origins were studied, and a common pattern of organization of nif -homologous sequences was found. Initial characterization of nif sequences in R. phaseoli has revealed the presence of stable reiterations of DNA sequences including nif sequences, and suggests that at least some of these sequences are located on a large plasmid.