The genera and , which constitute the family , are difficult to differentiate on the basis of phenotypic and chemotaxonomic attributes. DNA-rRNA hybridization studies have indicated that some spp. have the same level of relationship to the type species of the genus as some spp. A phylogenetic analysis of the 16S rRNA sequences of seven members of the indicated that the members of the genera and do not form separate monophyletic subgroups, confirming the lack of any phylogenetic support for retention of these taxa as separate genera. A phylogenetic analysis of the 16S rRNA sequence of confirmed that this species belongs in the All of the members of the examined and possess a cytosine residue at position 486 ( numbering), which is an extremely rare attribute among the prokaryotes and has been reported in only one other species, Several other signature characteristics which define this group in the gamma subclass of the are identified. The Jukes-Cantor distances between members of the family , including , range from 0.086 to 0.000 (the levels of similarity between the 16S rRNA sequences range from 92.6 to 100%). The members of the genera , and form a monophyletic group and share common chemotaxonomic and phenotypic characteristics. Subgroups containing members of the genera , and cannot be resolved on the basis of phylogenetic, chemotaxonomic, or phenotypic data. Our data indicate that the members of the genera , and should be united in a single genus.


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