The alpha (α) proteobacteria comprise an important group which has contributed seminally to many aspects of the history of life. The origin of mitochondria via the endosymbitotic capture of an α-proteobacteria is well established. There is also strong evidence indicating that the ancestral eukaryotic cell itself may have originated via a fusion, or long-term symbiotic association between one or more α-proteobacteria and an archaebacteria. The symbiosis between α-proteobacteria (viz. Rhizobiaceae species) and plant root nodules plays a central role in the fixation of atmospheric nitrogen by plants. Additionally, many α-proteobacterial species (viz. Rickettsiales, Brucella, Bartonella) are major human and animal pathogens. The α-proteobacteria exhibit enormous diversity in their morphological and metabolic characteristics and they are presently recognized solely on the basis of their branching pattern in the 16S rRNA trees. This group is presently given the rank of a Class or subdivision within the Proteobacteria phylum. Besides their branching pattern in phylogenetic trees, no reliable phenotypic or molecular characteristic is known that is uniquely shared by these bacteria. Described below are a large number of signatures in widely distributed proteins that are either uniquely shared by all α-proteobacteria, or certain subgroups (i.e. families or orders) among them. These signatures provide novel and definitive molecular means for distinguishing α-proteobacteria and its subgroups from all other bacteria. Based on the distribution of these signatures in different α-proteobacteria a detailed model for the evolutionary relationships among α-proteobacteria has been developed. Further, signature sequences in many proteins provide evidence that α-proteobacteria form a distinct main branch of the bacterial tree, which should be recognized as one of the main phylum of Bacteria.
|Branching order of α-proteobacteria based on conserved indels [JPEG]||Branching order of α-proteobacteria based on proteins specific to alpha-proteobacteria [PDF]|
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