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Somatic antigen groups and minimal somatic antigen constitution of 28 strains of Rhizobium japonicum were determined on the basis of their somatic cross-reactions and agglutinin absorptions. A total of 17 serological groups and 24 antigens was proposed to account for the reactions obtained. Some groups consisted of a complex of three or four strains while others comprised only one strain. Still others were composed of strains which were common to two serological groups. The antigenic complexity of rhizobial cells and the possibility of using "key-antisera" and polyvalent mixtures of "key-antisera" are discussed in relation to the classification of large numbers of field or greenhouse nodule isolates in Rhizobium competition studies.

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Nearly complete and short partial 16S rRNA sequences were derived from PCR-amplified ribosomal DNAs of Bradyrhizobium japonicum USDA 136 and USDA 110 and five strains of bacteriochlorophyll-synthesizing bacteria isolated from stem nodules of Aeschynomene indica and other Aeschynomene species growing in different geographic regions, including India, The Philippines and North America. We confirmed that the five stem-nodulating strains examined synthesize bacteriochlorophyll a, and the absorption spectra of methanolextracted cells contained a major absorbance peak at 770 nm. Strains isolated on different continents and from different Aeschynomene species were found to be phylogenetically homogeneous and exhibited levels of sequence similarity of more than 99%o. The bacteriochlorophyll-synthesizing rhizobia, Bradyrhizobium japonicum, Blastobacter denitrificans, Afipia felis, and Rhodopseudomonas palustris exhibited levels of sequence similarity of 97% or greater and belong to a distinct line of descent within the alpha-2 subdivision of the Proteobacteria. Variable regions between positions 995 and 1045 provide potential target sites for design of a probe that is able to distinguish the photosynthetic rhizobia from closely related taxa.

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