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Organization and expression of Rhizobium meliloti nitrogen fixation genes.

The boundaries of a nif gene cluster in Rhizobium meliloti were determined by Tn5 mutagenesis. These genes are clustered within a 14- to 15-kilobase (kb) region that includes the nitrogenase structural genes. Sequences within 10 kb on either side of this region are devoid of symbiotically essential gene function. RNA blot analysis identified a 5- to 6-kb transcript that corresponds to the nitrogenase structural gene operon. The 5' end of this transcript and its polarity were determined by nuclease S1 mapping. The 5' end of another nif transcript was also identified by nuclease S1 mapping. The promoter regions for these two nif transcripts control transcription in divergent directions and are separated by 1.9 kb of symbiotically unessential DNA. One Tn5 insertion within the nitrogenase operon did not create a polar mutation. The implications of this finding and the overall emerging picture of the genetic organization of this nif region are discussed.