Conservation of extended promoter regions of nodulation genes in Rhizobium

A 47-base-pair (bp) conserved sequence in the 5'-flanking regions of three transcriptional units coding for nodulation functions (nodABC, nodEFG, and nodH) has been identified in Rhizobium meliloti strain 41. The conserved region contains subsequences of 7 bp, 5 bp, and 25 bp. The conserved 25-bp sequence was synthesized and used as a hybridization probe; three additional copies of the sequence were identified in R. meliloti 41; all three were localized in the 135-kb nod/nif region of the symbiotic megaplasmid. Nucleotide sequence analysis of the six regions revealed that all contained the 47-bp conserved sequence but, with one exception, adjacent DNA regions did not have long conserved DNA stretches. The position of the 47-bp region was about 200-240 bp upstream of the translational start codons of the three nod genes. This conserved sequence is present in several other Rhizobium species and located adjacent to nod genes. We have demonstrated the involvement of this sequence in the expression of nodulation functions, which suggests that these extended promoter regions may have a role in the coordinated regulation of nodulation genes.