Candidate phyla radiation explained

The candidate phyla radiation (also referred to as CPR group) is a large evolutionary radiation of bacterial lineages whose members are mostly uncultivated and only known from metagenomics and single cell sequencing. They have been described as nanobacteria (not to be confused with non-living nanoparticles of the same name) or ultra-small bacteria due to their reduced size (nanometric) compared to other bacteria.

Originally (circa 2016), it has been suggested that CPR represents over 15% of all bacterial diversity and may consist of more than 70 different phyla.[1] However, the Genome Taxonomy Database (2018) based on relative evolutionary divergence found that CPR represents a single phylum,[2] with earlier figures inflated by the rapid evolution of ribosomal proteins.[3] CPR lineages are generally characterized as having small genomes and lacking several biosynthetic pathways and ribosomal proteins. This has led to the speculation that they are likely obligate symbionts.[4] [5]

Earlier work proposed a superphylum called Patescibacteria which encompassed several phyla later attributed to the CPR group.[6] Therefore, Patescibacteria and CPR are often used as synonyms.[7] The former name is not necessarily obsolete: for example, the GTDB uses this name because they consider the CPR group a phylum.[2]

Characteristics

Although there are a few exceptions, members of the candidate phyla radiation generally lack several biosynthetic pathways for several amino acids and nucleotides. To date, there has been no genomic evidence that indicates that they are capable of producing the lipids essential for cell envelope formation. Additionally, they tend to lack complete TCA cycles and electron transport chain complexes, including ATP synthase. This lack of several important pathways found in most free-living prokaryotes indicates that the candidate phyla radiation is composed of obligate fermentative symbionts.[8]

Furthermore, CPR members have unique ribosomal features. While the members of CPR are generally uncultivable, and therefore missed in culture-dependent methods, they are also often missed in culture-independent studies that rely on 16S rRNA sequences. Their rRNA genes appear to encode proteins and have self-splicing introns, features that are rarely seen in bacteria, although they have previously been reported.[9] Owing to these introns, members of CPR are not detected in 16S-dependent methods. Additionally, all CPR members are missing the L30 ribosomal protein, a trait that is often seen in symbionts.

Many of its characteristics are similar or analogous to those of ultra-small archaea (DPANN).[5]

Phylogeny

The Candidate phyla radiation was found to be the most basal-branching lineage in bacteria according to some early phylogenetic analyses of this group based on ribosomal proteins and protein family occurrence profiles. These studies found the following phylogeny between phyla and superphyla. The superphyla are shown in bold.[5] [4]

However, several recent studies have suggested that the CPR belongs to Terrabacteria and is more closely related to Chloroflexota.[10] [11] [12] The evolutionary relationships that are typically supported by these studies are as follows.

Provisional taxonomy

Because many CPR members are uncultivable, they cannot be formally put into the bacterial taxonomy, but a number of provisional, or Candidatus, names have been generally agreed on.[6] [13] As of 2017, two superphyla are generally recognized under CPR, Parcubacteria and Microgenomates.[1] The Phyla under CPR include:

The current phylogeny is based on ribosomal proteins (Hug et al., 2016).[4] Other approaches, including protein family existence and 16S ribosomal RNA, produce similar results at lower resolutions.[14] [1]

See also

External links

Notes and References

  1. Danczak RE, Johnston MD, Kenah C, Slattery M, Wrighton KC, Wilkins MJ . Members of the candidate phyla radiation are functionally differentiated by carbon- and nitrogen-cycling capabilities . Microbiome . 5 . 1 . 112 . September 2017 . 28865481 . 5581439 . 10.1186/s40168-017-0331-1 . free .
  2. Parks . Donovan . Chuvochina . Maria . Waite . David . Rinke . Christian . Skarshewski . Adam . Chaumeil . Pierre-Alain . Hugenholtz . Philip . A standardized bacterial taxonomy based on genome phylogeny substantially revises the tree of life . Nature Biotechnology . 27 August 2018 . 36 . 10 . 996–1004 . 10.1038/nbt.4229 . 30148503 . 52093100 . 13 January 2021.
  3. Parks . Donovan H. . Rinke . Christian . Chuvochina . Maria . Chaumeil . Pierre-Alain . Woodcroft . Ben J. . Evans . Paul N. . Hugenholtz . Philip . Tyson . Gene W. . Recovery of nearly 8,000 metagenome-assembled genomes substantially expands the tree of life . Nature Microbiology . November 2017 . 2 . 11 . 1533–1542 . 10.1038/s41564-017-0012-7 . 28894102 . free.
  4. Hug LA, Baker BJ, Anantharaman K, Brown CT, Probst AJ, Castelle CJ, Butterfield CN, Hernsdorf AW, Amano Y, Ise K, Suzuki Y, Dudek N, Relman DA, Finstad KM, Amundson R, Thomas BC, Banfield JF . 6 . A new view of the tree of life . Nature Microbiology . 1 . 5 . 16048 . April 2016 . 27572647 . 10.1038/nmicrobiol.2016.48 . free .
  5. Castelle CJ, Banfield JF . Major New Microbial Groups Expand Diversity and Alter our Understanding of the Tree of Life . Cell . 172 . 6 . 1181–1197 . March 2018 . 29522741 . 10.1016/j.cell.2018.02.016 . free .
  6. Rinke C . Nature . 2013 . 499 . 7459 . 431–7 . Insights into the phylogeny and coding potential of microbial dark matter . 23851394 . 10.1038/nature12352. etal. 2013Natur.499..431R . free . 10453/27467 . free .
  7. 10.3389/fmicb.2020.01848. Ancestral Absence of Electron Transport Chains in Patescibacteria and DPANN. 2020. Beam. Jacob P.. Becraft. Eric D.. Brown. Julia M.. Schulz. Frederik. Jarett. Jessica K.. Bezuidt. Oliver. Poulton. Nicole J.. Clark. Kayla. Dunfield. Peter F.. Ravin. Nikolai V.. Spear. John R.. Hedlund. Brian P.. Kormas. Konstantinos A.. Sievert. Stefan M.. Elshahed. Mostafa S.. Barton. Hazel A.. Stott. Matthew B.. Eisen. Jonathan A.. Moser. Duane P.. Onstott. Tullis C.. Woyke. Tanja. Stepanauskas. Ramunas. Frontiers in Microbiology. 11. 1848. 33013724. 7507113. free.
  8. Brown CT, Hug LA, Thomas BC, Sharon I, Castelle CJ, Singh A, Wilkins MJ, Wrighton KC, Williams KH, Banfield JF . 6 . Unusual biology across a group comprising more than 15% of domain Bacteria . Nature . 523 . 7559 . 208–11 . July 2015 . 26083755 . 10.1038/nature14486 . 2015Natur.523..208B . 1512215 . 4397558 .
  9. Belfort M, Reaban ME, Coetzee T, Dalgaard JZ . Marlene Belfort. Prokaryotic introns and inteins: a panoply of form and function . Journal of Bacteriology . 177 . 14 . 3897–903 . July 1995 . 7608058 . 177115 . 10.1128/jb.177.14.3897-3903.1995 .
  10. Coleman GA, Davín AA, Mahendrarajah TA, Szánthó LL, Spang A, Hugenholtz P, Szöllősi GJ, Williams TA . A rooted phylogeny resolves early bacterial evolution . Science . 2021 . 372 . 6542 . 10.1126/science.abe0511 . 33958449. 233872903 . 1983/51e9e402-36b7-47a6-91de-32b8cf7320d2 . free .
  11. Martinez-Gutierrez CA, Aylward FO . Phylogenetic signal, congruence, and uncertainty across bacteria and archaea . Molecular Biology and Evolution . 2021 . 38 . 12 . 5514–5527 . 10.1093/molbev/msab254 . 34436605. 8662615 .
  12. Taib N, Megrian D, Witwinowski J, Adam P, Poppleton D, Borrel G, Beloin C, Gribaldo S . Genome-wide analysis of the Firmicutes illuminates the diderm/monoderm transition . Nature Ecology and Evolution . 2020 . 4 . 12 . 1661–1672 . 10.1038/s41559-020-01299-7 . 33077930. 224810982 .
  13. Web site: Sayers. Patescibacteria group . National Center for Biotechnology Information (NCBI) taxonomy database . 2021-03-20.
  14. Méheust . Raphaël . Burstein . David . Castelle . Cindy J. . Banfield . Jillian F. . The distinction of CPR bacteria from other bacteria based on protein family content . Nature Communications . 13 September 2019 . 10 . 1 . 4173 . 10.1038/s41467-019-12171-z. 31519891 . 6744442 . 2019NatCo..10.4173M . free .

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