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

By heterologous hybridization, we have identified the common nodulatio n genes nodBCIJ of Rhizobium sp, strain N33 within a 8.2-kb PstI fragm ent, The nodBCIJ genes are located within a 4,620-bp region which also included a consensus nod box promoter. The four open reading frames c oding for the nodBCIJ genes contain 657, 1,353, 915, and 789 nucleotid es, respectively, We found that the nodA gene was not adjacent to the nodB gene, unlike the situation in many rhizobia, The DNA of the nodBC IJ genes of Rhizobium sp, strain N33 were found to be homologous to th e corresponding genes of other rhizobia except for the 3'-coding regio n of the nodC gene, The deduced NodC protein was the longest of the rh izobia 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 Astr agalus cicer, By contrast, mutations in the nodI and nodJ genes produc ed a Nod(+) phenotype with a reduced number of nodules on the temperat e hosts, Nodules formed on Onobrychis viciifolia by either nom or nodJ mutants were approximately 10 times smaller than nodules formed by th e wild type strain: this reduction in nodule size was not observed on Astragalus cicer.