Based on the partial nucleotide sequence analysis of 16S ribosomal ribonucleic acid (rRNA), presence of unique sphingoglycolipids in cellular lipid, and the major type of ubiquinone (Q10), we propose Sphingomonas gen. nov. with the type species Sphingomonas paucimobilis (Holmes et al, 1977) comb. nov. From the homology values of deoxyribonucleic acid-deoxyribonucleic acid hybridization and the phenotypic characteristics, three new species, Sphingomonas parapaucimobilis, Sphingomonas yanoikuyae, Sphingomonas adhaesiva, and one new combination, Sphingomonas capsulata, are described. S. parapaucimobilis JCM 7510 (= GIFU 11387), S. yanoikuyae JCM 7371 (= GIFU 9882), and S. adhaesiva JCM 7370 (= GIFU 11458) are designated as the type strains of the three new species. Emended description of the type strain of S. capsulata is presented.

A bacterium isolated from a polluted stream, capable of metabolizing biphenyl, naphthalene, phenanthrene, and higher-molecular-weight polycyclic aromatic hydrocarbons (D. Gibson, V. Mahadevan, D. Jerina, H. Yagi, and H. Yeh, Science 189:295-297, 1975), was previously identified as Beijerinckia sp. strain B1. In this investigation, 16S rRNA gene sequencing, biochemical tests, fatty acid methyl ester analysis, polyacrylamide gel electrophoresis of protein, and DNA-DNA hybridization were used to determine the taxonomic relationship of Beijerinckia sp. strain B1. The sequence of the 16S rRNA gene of B1 was identical to that of Sphingomonas yanoikuyae ATCC 51230T. The biochemical tests, fatty acid analysis, and sodium dodecyl sulfate-polacrylamide gel electrophoresis profile of soluble proteins of strain B1 showed results similar to those of S. yanoikuyae. DNA-DNA hybridization indicated that B1 and S. yanoikuyae ATCC 51230T are 75% homologous at the DNA level. We propose that Beijerinckia sp. strain B1 be reclassified as S. yanoikuyae.

Phylogenetic analyses of 16S rRNA gene sequences by distance matrix and parsimony methods indicated that the currently known species of the genus Sphingomonas can be divided into four clusters. Some chemotaxonomic and phenotypic differences were noted among these clusters. Three new genera, Sphingobium, Novosphingobium and Sphingopyxis, are proposed in addition to the genus Sphingomonas sensu stricto. The genus Sphingobium is proposed to accommodate Sphingomonas chlorophenolica, Sphingomonas herbicidovorans and Sphingomonas yanoikuyae. The genus Novosphingobium is proposed for Sphingomonas aromaticivorans, Sphingomonas capsulata, Sphingomonas rosa, Sphingomonas stygia, Sphingomonas subarctica and Sphingomonas subterranea. Sphingomonas macrogoltabidus and Sphingomonas terrae are reclassified in the genus Sphingopyxis. The type species of Sphingobium, Novosphingobium and Sphingopyxis are Sphingobium yanoikuyae, Novosphingobium capsulatum and Sphingopyxis macrogoltabida, respectively.

The 16S rDNA sequence similarities between the type strains of Sphingomonas paucimobilis and 32 other Sphingomonas species range from 90.2 to 99.6%. It might be possible to divide the genus into several new genera according to a dendrogram drawn from 16S rDNA sequence similarity. However, the phenotypic and biochemical information needed to define clusters of strains representing distinct genera within this group of organisms was not previously available. Although the cellular lipids of type strains of all 28 Sphingomonas species tested contained glucuronosyl-(1 --> 1)-ceramide together with 2-hydroxymyristic acid, other molecular species of sphingoglycolipids were distributed randomly. Sphingomonas natatoria and Sphingomonas ursincola, bacteriochlorophyll a-containing, gram-negative facultative phototrophs, belong to the cluster of the genus Sphingomonas. Other phototrophic Porphyrobacter and Erythrobacter species in the Sphingomonadaceae were classified into a cluster different from the genus Sphingomonas, as reported previously. None of the physiological and biochemical characteristics considered, including cellular lipids and fatty acid composition, provided evidence for the division of the current genus Sphingomonas. It is therefore concluded that the genus Sphingomonas should remain undivided at this time. The species of three recently proposed genera, Sphingobium, Novosphingobium and Sphingopyxis, in conjunction with Blastobacter ursincola, are junior objective synonyms of species of the genus Sphingomonas.