Sequence and Mutational Analysis of the Common nodBCIJ Region of Rhizobium sp. (Oxytropis arctobia) Strain N33, a Nitrogen-Fixing Microsymbiont of Both Arctic and Temperate Legumes. Jean Cloutier. Recherche en sciences de la vie et de la sante, Pavilion Charles-Eugene-Marchand, Universite Laval, Quebec (Quebec), Canada G1K 7P4. Serge Laberge (2), Danielle Prevost (2), and Hani Antoun (2) (1) Recherche en sciences de la vie et de la sante, Pavilion Charles-Eugene-Marchand, Universite Laval, Quebec (Quebec), Canada G1K 7P4, and (2) Centre de recherche et de developpement sur les sols et les grandes cultures, Agriculture et Agroalimentaire Canada, 2560, boul. Hochelaga, Sainte-Foy (Quebec), Canada G1V2J3. MPMI 9:523-531. Accepted 19 April 1996. Copyright 1996 The American Phytopathological Society.

By heterologous hybridization, we have identified the common nodulation genes nodBClJ of Rhizobium sp. strain N33 within a 8.2-kb Pstl fragment. The nodBClJ genes are located within a 4,620-bp region which also included a consensus nod box promoter. The four open reading frames coding for the nodBClJ genes contain 657, 1,353, 915, and 789 nucleotides, respectively. We found that the nodA gene was not adjacent to the nodB gene, unlike the situation in many rhizobia. The DNA of the nodBCIJ genes of Rhizobium sp. strain N33 were found to be homologous to the corresponding genes of other rhizobia except for the 3'-coding region of the nodC gene. The deduced NodC protein was the longest of the rhizobia except Bradyrhizobium japonicum. Tn5 mutagenesis of the common nod region of strain N33 revealed that the nodBC genes were essential for nodulation on their temperate hosts Onobrychis viciifolia and Astragalus cicer. By contrast, mutations in the nodI and nodi genes produced a Nod+ phenotype with a reduced number of nodules on the temperate hosts. Nodules formed on Onobrychis viciifolia by either nodI or nodi mutants were approximately 10 times smaller than nodules formed by the wild type strain: this reduction in nodule size was not observed on Astragalus cicer