Identification of NolR, a negative transacting factor controlling the nod regulon in Rhizobium meliloti

Eva Kondorosi, Michèle Pierre, Michèle Cren, Ursula Haumann, Michel Buiré, Beate Hoffmann, Jeff Schell, Adam Kondorosi

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65 Citations (Scopus)


In Rhizobium meliloti, expression of the nodulation genes (nod and nol genes) is under both positive and negative controls. These genes are activated by the products of the three related nodD genes, in conjunction with signal molecules from the host plants. We showed that negative regulation is mediated by a repressor protein, binding to the overlapping nodD1 and nodA as well as to the nodD2 promoters. The encoding gene, termed nolR, was identified and cloned from strain 41. By subcloning, deletion and Tn5 mutagenesis, a region of 594 base-pairs was found to be necessary and sufficient for repressor production in strains of R. meliloti lacking the repressor or in Escherichia coli. Sequence analysis revealed that nolR encodes a 13,349 Da protein, which is in agreement with the molecular weight of the NolR protein, determined after purification by affinity chromatography, utilizing long synthetic DNA multimers of the 21 base-pair conserved repressor-binding sequence. Our data suggest that the native NolR binds to the operator site in dimeric form. The nolR contains a helix-turn-helix motif, which shows homology to the DNA-binding sequences of numerous prokaryotic regulatory proteins such as the repressor XylR or the activator NodD and other members of the LysR family. Comparison of the putative DNA-binding helix-turn-helix motifs of a large number of regulatory proteins pointed to a number of novel regularities in this sequence. Hybridizations with an internal nolR fragment showed that sequences homologous to the nolR gene are present in all R. meliloti isolates tested, even in those that do not produce the repressor. In another species, such as Rhizobium leguminosarum, where NodD is autoregulated, however, such sequences were not detected.

Original languageEnglish
Pages (from-to)885-896
Number of pages12
JournalJournal of molecular biology
Issue number4
Publication statusPublished - Dec 20 1991


  • Rhizobium meliloti
  • nod gene expression
  • nod regulon
  • nod repressor
  • nolR

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology

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