Clustering of nitrogen fixation (nif) genes in Rhizobium meliloti

A cloned 17.3-kilobase region of the Rhizobium meliloti genome with homology to the Klebsiella pneumoniae nitrogenase structural genes was studied. Limits on the extent of homology were determined. Transposon mutagenesis of this region of the genome verified that it contained functional nif genes, Some transposon insertions resulted in a defective symbiotic phenotype, whereas others had no noticeable effect on symbiotic competence. The relative position of insertions yielding these two phenotypic classes suggested that at least three distinct units of gene expression are present in this region. Hybridization of RNA from alfalfa root nodules and from vegetatively grown Rhizobium to this cloned DNA showed that at least 11.1 kilobases of the region was transcribed actively and that transcription was specific for the symbiotic state.