The nodulation genes of Mesorhizobium sp. (Astragalus sinicus) strain 7653R were cloned by functional complementation of Sinorhizobium meliloti nod mutants. The common nod genes, nodD, nodA, and nodBC, were identified by heterologous hybridization and sequence analysis. The nodA gene was found to be separated from nodBC by approximately 22 kb and was divergently transcribed. The 2.0-kb nodDBC region was amplified by PCR from 24 rhizobial strains nodulating A. sinicus, which represented different chromosomal genotypes and 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 indicated that seven strains representing different 16S and 23S ribosomal DNA genotypes had identical nodA sequences. These data indicate that, whereas microsymbionts of A. sinicus exhibit chromosomal diversity, their nodulation genes are conserved, supporting the hypothesis of horizontal transfer of nod genes among diverse recipient bacteria.Rhizobia are soil bacteria that can form nodules, in which they fix nitrogen, on leguminous plants in a host-specific manner. Nodulation (nod) genes have been identified that control the specific infection and nodulation of the plant hosts. The initial infection event is regulated by a NodD protein or proteins which activate the transcription of other nod genes in the presence of host-produced flavonoids (12, 25, 37). The nod-ABC genes are called common nod genes because they are present in all rhizobia. Other nod genes, such as nodFE, nodH, nodSU, and nodZ (12,25,37), are present in various combinations in rhizobial species and are called host-specific nod genes.Expression of common and host-specific nod genes results in the production of lipochitooligosaccharides (Nod factors) that act as morphogenic signal molecules on specific legume hosts (12, 37). All Nod factors have a ␤-1,4-linked N-acetyl glucosamine oligosaccharide backbone ranging in length from 3 to 5 residues and substituted for by an N-acyl chain at the nonreducing end and other chemical groups on the glucosamine residues. The common nodABC gene products are involved in the synthesis of the N-acylated oligosaccharide core, while the host-specific nod gene products are involved in the decoration of this backbone with substitutions that confer plant specificity. The nodABC genes encode an acyltransferase, a chitin oligosaccharide deacetylase, and a chitin oligosaccharide synthase, respectively (3, 33). The common nod genes are also involved in determining host range specificity to some extent. For example, different NodA proteins recognize and transfer different fatty acid chains to the chitooligosaccharide chain, the length of which is determined by NodC (11, 27, 32). The common nodABC genes are essential for nodule formation. Mutation in any of them abolishes the ability to produce Nod factors and results in a nonnodulating (Nod Ϫ ) phenotype (12). Astragalus sinicus L. (Chinese ...

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Two hundred and four isolates of rhizobia were sampled from root nodules of Astragalus sinicus grown in rice fields of six southern provinces of China. Genotypic diversity was determined by Southern hybridization using nodDBC genes as a probe, restriction fragment length polymorphism (RFLP) analysis of PCR-amplified 16S-23S rDNA intergenic spacers (IGS), and plasmid profile. Our results show that rhizobia associated with A. sinicus were very diverse, and 10 genotypes were resolved within the previously identified dominant 16S rDNA type. Diversity levels varied greatly between different geographical locations. The same nod gene genotypes were harbored by distinct chromosomal types, suggesting that lateral plasmid transfer occurred during the evolution process.

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