The common nodulation genes of Astragalus sinicus rhizobia are conserved despite chromosomal diversity

The nodulation genes of Mesorhizobium sp, (Astragalus sinicus) strain 7653R were cloned by functional complementation of Sinorhizobium meliloti nod mu tants. The common nod genes, nodD, nodA, and nodBC, were identified by hete rologous hybridization and sequence analysis. The nodA gene was found to be separated from nodBC by approximately 22 kb and was divergently transcribe d. The 2.0-kb nonDBC region was amplified by PCR from 24 rhizobial strains nodulating A. sinicus, which represented different chromosomal genotypes an d geographic origins. No polymorphism was found in the size of PCR products , suggesting that the separation of nodA from nodBC is a common feature of A. sinicus rhizobia. Sequence analysis of the PCR-amplified nodA gene indic ated that seven strains representing different 16S and 23S ribosomal DNA ge notypes had identical nodA sequences. These data indicate that, whereas mic rosymbionts of A. sinicus exhibit chromosomal diversity, their nodulation g enes are conserved, supporting the hypothesis of horizontal transfer of nod genes among diverse recipient bacteria.