Summary: Partial sequences of the 16S ribosomal RNA genes of eleven autotrophic ammonia-oxidizing bacteria were determined by PCR amplification from small amounts of heat-lysed biomass followed by direct sequencing of PCR products. The sequences were aligned with those of representative Proteobacteria and phylogenetic trees inferred using both parsimony and distance matrix methods. This confirmed that the autotrophic ammonia-oxidizers comprise two major lines of descent within the Proteobacteria. spp., , and strains of and were located in the beta-subdivision. The recovery of strains as a deep branch in the gamma-subdivision supported the RNA catalogue data which had indicated that the genus is polyphyletic. The autotrophic ammonia-oxidizing bacteria of the beta-Proteobacteria formed a coherent group which is interpreted as representing a single family. Within this clade, the genera and exhibited very high levels of homology in their 16S ribosomal RNA gene sequences and can be accommodated within a single genus. Separation of these genera is currently based entirely on gross morphological differences and these can now be considered more appropriate for the identification of species within this group. It is therefore proposed that and strains be reclassified in a single genus for which the name has priority.


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