A 260-bp segment of the DNA that encodes 16S rRNA, corresponding to positions 44 to 337 in the Escherichia coli sequence, was amplified by the polymerase chain reaction and sequenced frofn each of 13 bacteria (rhizobia and purple phototrophs) in the alpha subdivision of the class Proteobacteria. The phylogenetic tree calculated from differences in the sequenced segment conforms well to our expectations based on other previously published data. The sequence from BTAil (a recently described phototrophic symbiont of the legume Aeschynomene) and that from the free-living phototroph Rhodopseudomonas palustris both fall within the range of variation found among strains of the soybean symbiont Bradyrhizobium japonicum. This suggests that it would be appropriate to include all of these organisms in a single genus.

We determined the sequences for a 260-base segment amplified by the polymerase chain reaction (corresponding to positions 44 to 337 in the Escherichia coli 16s rRNA sequence) from seven strains of fast-growing soybean-nodulating rhizobia (including the type strains of Rhizobium fredii chemovar fredii, Rhizobium fredii chemovar siensis, Sinorhizobium fredii, and Sinorhizobium xinjiangensis) and broad-hostrange Rhizobium sp. strain NGR 234. These sequences were compared with the corresponding previously published sequences of Rhizobium leguminosarum, Rhizobium meliloti, Agrobacterium tumefaciens, Azorhizobium caulinodans, and Bradyrhizobium japonicum. All of the sequences of the fast-growing soybean rhizobia, including strain NGR 234, were identical to the sequence of R. rneliloti and similar to the sequence of R. leguminosarum. These results are discussed in relation to previous findings; we concluded that the fast-growing soybean-nodulating rhizobia belong in the genus Rhizobium and should be called Rhizobium fredii.In 1982 Keyser et al. described fast-growing rhizobia which produced nodules and fixed atmospheric nitrogen on the roots of soybeans and were isolated in the People's Republic of China (15). These bacteria had cultural and physiological properties that were typical of the fast-growing rhizobia classified in the genus Rhizobium (20,25,33), although their host range was similar to that of the slowgrowing species Bradyrhizobium japonicum (10, 25). The results of DNA-DNA hybridization studies indicated that the fast-growing soybean-nodulating (FGSN) rhizobia form two closely related groups but exhibit low levels of relationship to the type strains of other rhizobial species (23). Consequently, these organisms were placed in a new species, which was designated Rhizobium fredii (22). The results of serological and phylogenetic studies (13,19,29) indicated that R. fredii and Rhizobium meliloti are related.The work described above was based on 11 strains of fast-growing soybean rhizobia collected in 1982 (15). In 1988 Chen et al. (4) described the results of a numerical taxonomic study involving 9 of the 11 strains that were originally classified as R . fredii and 24 additional strains of fastgrowing soybean rhizobia. These organisms were compared with 25 named strains belonging to the genera Rhizobium, Bradyrhizobium, and Agrobacterium. Chen et al. (4) found little relationship between the FGSN rhizobia and the named strains and proposed that R. fredii should be reclassified in a new genus, Sinorhizobium.In this paper we describe partial sequences of 16s rRNA genes from representative strains of FGSN rhizobia that were obtained from W. X. Chen and from the U.S. Department of Agriculture. We compared these sequences with sequences from the corresponding regions in R. meliloti, Rhizobium leguminosarum, Agrobacterium tumefaciens, Azorhizobium caulinodans, and B. japonicum. Below we also discuss the evidence for and against classification of the FGSN rhizobia in a separate genus, Sinorhizobium; we concluded th...