Anoxyphotobacteria(Gibbons and Murray 1978) Murray 1988
PhotobacteriaGibbons and Murray 1978 (Approved Lists 1980)
AlphabacteriaCavalier-Smith 2002
Alphaproteobacteria orα-proteobacteria, also calledα-Purple bacteria in earlier literature, is aclass ofbacteria in thephylumPseudomonadota (formerly "Proteobacteria").[4] TheMagnetococcales andMariprofundales are considered basal or sister to theAlphaproteobacteria.[5][6] TheAlphaproteobacteria are highly diverse and possess few commonalities, but nevertheless share a common ancestor. Like allProteobacteria, its members aregram-negative, although some of its intracellular parasitic members lackpeptidoglycan and are consequently gram variable.[4][3]
TheAlphaproteobacteria are a diverse taxon and comprise severalphototrophic genera, several genera metabolising C1-compounds (e.g.Methylobacterium spp.), symbionts of plants (e.g.Rhizobium spp.),endosymbionts of arthropods (Wolbachia) and intracellularpathogens (e.g.Rickettsia). Moreover, the class is sister to theprotomitochondrion, the bacterium that was engulfed by the eukaryotic ancestor and gave rise to themitochondria, which are organelles in eukaryotic cells (seeEndosymbiotic theory).[1][7] A species of technological interest isRhizobium radiobacter (formerlyAgrobacterium tumefaciens): scientists often use this species to transfer foreign DNA into plant genomes.[8]Aerobic anoxygenic phototrophic bacteria, such asPelagibacter ubique, are alphaproteobacteria that are a widely distributed and may constitute over 10% of the open ocean microbial community.
Several points of disagreement muddy the recovery of thephylogenetic relationships among theAlphaproteobacteria clades from the genomic data. One such point centers on the placement of thePelagibacterales stemming from the large differences in gene content (e.g.genome streamlining inPelagibacter ubique) andGC-content between members of several orders.[1] Specifically, certain species withinPelagibacterales,Rickettsiales, andHolosporales possessAT-rich genomes, containing higher-assayed concentrations of adenine-thymine (AT) pairs than guanine-cytosine (GC) base pairs. While it could be a case ofconvergent evolution resulting in an artefactual clustering,[9][10][11] several studies disagree[1][12][13][14] and no consensus has been reached.
Furthermore, the GC-content ofribosomal RNA, the traditionalphylogenetic marker for prokaryotes, does not correlate well with the GC-content of the genome. For example, members of theHolosporales have a much higher ribosomal GC-content than members of thePelagibacterales andRickettsiales, though they are more closely related to species with high genomic GC-contents than to members of the latter two orders.[1]
Comparative analyses of thesequenced genomes have revealed manyconservedinsertion-deletions (indels) in widely distributed proteins and whole proteins (i.e.signature proteins) that are distinctive characteristics of either allAlphaproteobacteria, or their different main orders (viz.Rhizobiales,Rhodobacterales,Rhodospirillales,Rickettsiales,Sphingomonadales andCaulobacterales) and families (viz.Rickettsiaceae,Anaplasmataceae,Rhodospirillaceae,Acetobacteraceae,Bradyrhiozobiaceae,Brucellaceae andBartonellaceae).
These molecular signatures provide a means to circumscribe the taxonomic groups and to identify and assign new species accurately.[16] Phylogenetic analyses and conserved indels in large numbers of other proteins provide evidence thatAlphaproteobacteria have branched off later than most other phyla and classes ofBacteria exceptBetaproteobacteria andGammaproteobacteria.[17][18]
Other phylogenetic debates turn on the placement ofMagnetococcidae and the protomitochondrion.[19][20] There are some debates for the inclusion ofMagnetococcidae inAlphaproteobacteria. For example, an independent proteobacterial class ("Candidatus Etaproteobacteria") forMagnetococcidae has been proposed.[21][22] A recent phylogenomic study suggests the placement of the protomitochondrial clade betweenMagnetococcidae and all other alphaproteobacterial taxa,[5] which suggests an early divergence of the protomitochondrial lineage from the rest of alphaproteobacteria, except forMagnetococcidae. This phylogeny also suggests that the protomitochondrial lineage does not necessarily have a close relationship toRickettsidae.
^Chilton MD, Drummond MH, Merio DJ, Sciaky D, Montoya AL, Gordon MP, Nester EW (June 1977). "Stable incorporation of plasmid DNA into higher plant cells: the molecular basis of crown gall tumorigenesis".Cell.11 (2):263–271.doi:10.1016/0092-8674(77)90043-5.PMID890735.S2CID7533482.
^Bazylinski DA, Williams TJ, Lefèvre CT, Berg RJ, Zhang CL, Bowser SS, Dean AJ, Beveridge TJ (2012). "Magnetococcus marinus gen. nov., sp. nov., a marine, magnetotactic bacterium that represents a novel lineage (Magnetococcaceae fam. nov.;Magnetococcales ord. nov.) at the base of theAlphaproteobacteria".Int J Syst Evol Microbiol.63 (Pt 3):801–808.doi:10.1099/ijs.0.038927-0.PMID22581902.
^Gupta RS (2005). "Protein signatures distinctive of alpha proteobacteria and its subgroups and a model for alpha-proteobacterial evolution".Critical Reviews in Microbiology.31 (2):101–35.doi:10.1080/10408410590922393.PMID15986834.S2CID30170035.
^Ji B, Zhang SD, Zhang WJ, Rouy Z, Alberto F, Santini CL, et al. (March 2017). "The chimeric nature of the genomes of marine magnetotactic coccoid-ovoid bacteria defines a novel group of Proteobacteria".Environmental Microbiology.19 (3):1103–1119.doi:10.1111/1462-2920.13637.PMID27902881.S2CID32324511.
