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Conservation of noIR in the Sinorhizobium and Rhizobium Genera of the Rhizobiaceae Family

December 1998 , Volume 11 , Number  12
Pages  1,186 - 1,195

Ernö Kiss , 1 , 2 Peter Mergaert , 1 Boglàrka Olàh , 1 , 2 Attila Kereszt , 1 , 2 Christian Staehelin , 1 Andrea E. Davies , 3 J. Allan Downie , 3 Adam Kondorosi , 1 , 2 and Eva Kondorosi 1

1Institut des Sciences Végétales, CNRS, Avenue de la Terrasse, F-91198 Gif-sur-Yvette Cedex, France; 2Institute of Genetics, Biological Research Center, Hungarian Academy of Sciences, H-6701 Szeged, P.O. Box 521, Hungary; 3John Innes Centre, Colney, Norwich NR4 7UH, UK

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Accepted 4 August 1998.

In Sinorhizobium meliloti the NolR repressor displays differential negative regulation of nodulation genes and is required for optimal nodulation. Here, we demonstrate that the NolR function is not unique to S. meliloti but is also present in other species of the Rhizobiaceae family. DNA hybridization indicates the presence of nolR homologous sequences in species belonging to the Rhizobium and Sinorhizobium genera while no hybridization signal was detected in species from the Mesorhizobium, Bradyrhizo-bium, Azorhizobium, and Agrobacterium genera. We isolated the nolR gene from the Rhizobium leguminosarum bv. viciae strain TOM and showed that the TOM nolR gene acts similarly to S. meliloti nolR by repressing the expression of both the nodABCIJ and the nodD genes, resulting in decreased Nod factor production. The presence of a functional nolR gene in R. leguminosarum is correlated with an increased rate and extent of nodulation of pea. The conserved primary structure, the location of the DNA-binding domain, and the similar size of NolR proteins, compared with a family of small bacterial regulatory proteins including HlyU, SmtB, and the ArsR-type regulators, revealed that NolR belongs to this family.

© 1998 The American Phytopathological Society