A phylogenetic survey of budding, and/or prosthecate, non-phototrophic eubacteria: membership of Hyphomicrobium, Hyphomonas, Pedomicrobium, Filomicrobium, Caulobacter and "dichotomicrobium" to the alpha-subdivision of purple non-sulfur bacteria.

The phylogenetic position of various budding and/or or prosthecate Gram-negative eubacteria was determined by different methods. Members of the genera Hyphomicrobium, Filomicrobium, Pedomicrobium were investigated by 16S rRNA cataloguing, a 1373 nucleotide long portion of the 16S rRNA was sequenced from Hyphomicrobium vulgare and the 5S rRNAs were analyzed from two Hyphomicrobium strains, Hyphomonas polymorpha and Caulobacter crescentus. Comparison with published sequences indicated a membership of all of these organisms to the alpha subdivision of purple bacteria. While C. crescentus and Hyphomonas polymorpha constitute separate individual lines of descent, the position of a coherent cluster embracing Hyphomicrobium, Pedomicrobium and Filomicrobium is not yet settled. 16S rRNA cataloguing indicate the presence of a distinct line equivalent to other subgroups in its phylogenetic depth. 5S rRNA analysis, on the other hand, groups Hyphomicrobium vulgare and strain IFAM 1761 with members of subgroup alpha-2 (Rhodopseudomonas palustris, Nitrobacter winogradskyi and relatives). In contrast to the present classification, Pedomicrobium ferrugineum and Filimicrobium fusiforme are more closely related to certain Hyphomicrobium strains than these are related among each other. Budding mode of reproduction and prosthecate morphology are dominating morphological features of members of the alpha subdivision. These characteristics may gain diagnostic significance in a future formal description of this subdivision and its subgroups as a higher rank.