The phylum Deinococcus-Thermus consists of a single class and two orders:
The Deinococcales include two families, with three genera, Deinococcus, Deinobacterium and Truepera. Truepera radiovictrix is the earliest diverging member of the order. Within the order, Deinococcus forms a distinct monophyletic cluster with respect toDeinobacterium and Truepera species. The genus includes several species that are resistant to radiation; they have become famous for their ability to eat nuclear waste and other toxic materials, survive in the vacuum of space and survive extremes of heat and cold.
The Thermales include several genera resistant to heat placed within a single family, Thermaceae. Phylogenetic analyses demonstrate that within the Thermales, Meiothermus and Thermus species form a monophyletic cluster, with respect to Marinithermus, Oceanithermus, Vulcanithermus and Rhabdothermus that branch as outgroups within the order. This suggests that Meiothermus and Thermus species are more closely related to one another relative to other genera within the order. Thermus aquaticus was important in the development of the polymerase chain reaction where repeated cycles of heating DNA to near boiling make it advantageous to use a thermo-stable DNA polymerase enzyme.
Though these two groups evolved from a common ancestor, the two mechanisms of resistance appear to be largely independent.
Molecular signatures
Molecular signatures in the form of conserved signature indels and proteins have been found that are uniquely shared by all members belonging to the Deinococcus-Thermus phylum. These CSIs and CSPs are distinguishing characteristics that delineate the unique phylum from all other bacterial organisms, and their exclusive distribution is parallel with the observed differences in physiology. CSIs and CSPs have also been found that support order and family-level taxonomic rankings within the phylum. Some of the CSIs found to support order level distinctions are thought to play a role in the respective extremophilic characteristics. The CSIs found in DNA-directed RNA polymerase subunit beta and DNA topoisomerase I in Thermales species may be involved in thermophilicity, while those found in Excinuclease ABC, DNA gyrase, and DNA repair protein RadA in Deinococcales species may be associated with radioresistance. Two CSPs that were found uniquely for all members belonging to the Deinococcus genus are well characterized and are thought to play a role in their characteristic radioresistant phenotype. These CSPs include the DNA damage repair protein PprA the single-stranded DNA-binding protein DdrB. Additionally, some genera within this group, including Deinococcus, Thermus and Meiothermus, also have molecular signatures that demarcate them as individual genera, inclusive of their respective species, providing a means to distinguish them from the rest of the group and all other bacteria. CSIs have also been found specific for Truepera radiovictrix.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature and National Center for Biotechnology Information
The two Meiothermus species were sequenced under the auspices of the Genomic Encyclopedia of Bacteria and Archaea project, which aims at sequencing organisms based on phylogenetic novelty and not on pathogenicity or notoriety. Currently, the genome of Thermus aquaticus Y51MC23 is in the final stages of assembly by the DOE Joint Genome Institute
