Conservation of nif- and species-specific domains within repeated promoter sequences from fast-growing Rhizobium species.

In the fast-growing Rhizobium species, repeated DNA sequences, which include the promoter region of the nif HDK operon have been described. These repeated sequences are promoters which specifically activate transcription in the endosymbiotic state. Hybridization analysis of these sequences from R. trifolii has revealed that they may be involved in the species-specific activation of the various genes whose transcription they promote. Comparative analysis of various copies of these repeated sequences, from R. trifolii (the clover symbiont) and R. meliloti (the alfalfa symbiont), reveals the presence of domains of intra- and interspecific conservation within the promoter regions. We suggest that these promoter elements represent sites which are involved in the species-specific and general, nif-specific activation of Rhizobium symbiotic genes.