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. In Rhizobium meliloti, genes required for nodulation and nitrogen fixation (nod and fix) are located on a megaplasmid (2-4). A 135-kilobase (kb) segment of this plasmid in R. meliloti strain 41 was shown to carry all essential nodulation genes and some of the fixation genes (5). The essential nodulation genes are clustered on two EcoRI fragments. An 8.5-kb fragment carries the interspecifically highly conserved common nodulation genes (nodABC, ref. 6) and a newly identified gene, nodD (ref. 7; unpublished results).The second nod fragment of 6.8 kb codes for genes necessary for host-specific nodulation (hsn, ref. 6). Transposon 5 (Trn5) mutagenesis and nucleotide sequence analysis of this region have identified four genes involved in alfalfa nodulation, organized into probably two transcription units (unpublished results).The number of identified nod genes is increasing, and it is apparent that most or probably all of these are turned on specifically during the symbiotic process. It is likely that the control of expression of the nodulation genes is tightly coordinated, and this may be reflected in the conservation of specific regulatory sequences in the promoter regions of the nod operons.In this paper we present evidence identifying a 47-base pair (bp) conserved sequence in the 5' region of a set of nodulation genes of R. meliloti 41. The 47-bp sequence is situated ca. Fig. 3 and ref. 2). AK1746, AK1745, AK1711, and BH91 are TnS mutants in the hsn region of AK631 isolated in this laboratory (see Fig. 4). AK3216 and AK3217 are insertion mutants of AK631 (see Fig. 4). R. meliloti L5-30 and its deletion derivative R. meliloti L5-22 were obtained from J. Denarie. Wild-type Rhizobium strains R. trifolii RS10151, R. phaseoli RCR3610, and R.japonicum USDA122 were provided by F. Rodriguez-Quinofies, W. J. Broughton, and H. Evans, respectively. Rhizobium sp. NGR234, and R. legumin...

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The nucleotide sequence of a 2224 bp region of the Escherichia coli chromosome that carries the LexA regulated recN gene has been determined. A region of 1701 nucleotides encoding a polypeptide of 567 amino acids with a predicted molecular weight of 63,599 was identified as the most probable sequence for the recN structural gene. The proposed initiation codon is preceded by a reasonable Shine-Dalgarno sequence and a promoter region containing two 16 bp sequences, separated by 6 bp, that match the consensus sequence (SOS box) for binding LexA protein. DNA fragments containing this putative promoter region are shown to bind LexA in vitro and to have LexA-regulated promoter activity in vivo. The amino acid sequence of RecN predicted from the DNA contains a region that is homologous to highly conserved sequences found in several DNA repair enzymes and other proteins that bind ATP. A sequence of 9 amino acids was found to be homologous to a region of the RecA protein of E. coli postulated to have a role in DNA/nucleotide binding.

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