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ScienceWeek
EVOLUTIONARY BIOLOGY: ON THE HYDROGENOSOME
The following points are made by Michael W. Gray (Nature 2005 434:29):
1) Hydrogenosomes are double-membraned subcellular structures that generate hydrogen while making the energy-storage compound ATP. They are found in certain eukaryotic (nucleus-containing) microbes that inhabit oxygen-deficient environments[1]. The evolution of the hydrogenosome has remained obscure, mainly because these organelles seemed not to have a genome[2,3] --until now. Boxma et al[4] report the characterization of what seems to be an authentic hydrogenosomal genome in the anaerobic microbe Nyctotherus ovalis, an inhabitant of the termite hindgut.
2) In eukaryotes that live in oxygen-rich (aerobic) environments, organelles called mitochondria are responsible for making ATP. Although an evolutionary relationship between hydrogenosomes and mitochondria has been postulated, this hypothesis remains contentious[2,3]. Mitochondria contain a small genome (mtDNA) that retains traces of their evolutionary origin from a bacterial symbiont[5]. The hydrogenosomal DNA isolated by Boxma et al exhibits hallmarks of a bona fide mitochondrial genome.
3) Adding to this story are recent studies that probe the evolutionary history of the hydrogenosome from another anaerobic microbe, the parasite Trichomonas vaginalis. The absence of a hydrogenosomal genome in this organism makes it a challenging task to infer the origin of its hydrogenosome. Indeed, on this point, researchers come to rather different conclusions, even though they analyse the same Trichomonas hydrogenosomal proteins.
4) In animals and fungi, the mitochondrial genome encodes a small number of essential inner-membrane proteins (components of respiratory complexes I-IV and complex V, a specialized type of ATP-synthesizing enzyme) that function in electron transport and ATP production[5]. In addition, mtDNA specifies the RNA components of the mitochondrial protein-synthesis system and, in plants and many algae and protozoa, some of the proteins of this system too[5].
References (abridged):
1. Müller, M. J. Gen. Microbiol. 139, 2879-2889 (1993)
2. Embley, T. M. et al. IUBMB Life 55, 387-395 (2003)
3. Dyall, S. D., Brown, M. T. & Johnson, P. J. Science 304, 253-257 (2004)
4. Boxma, B. et al. Nature 434, 74-79 (2005)
5. Gray, M. W., Burger, G. & Lang, B. F. Science 283, 1476-1481 (1999)
Nature http://www.nature.com/nature
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Related Material:
A HYDROGENOSOME WITH A GENOME
Notes by ScienceWeek:
Certain *anaerobic *protozoa and *fungi possess membrane-bound organelles known as "hydrogenosomes". These organelles are approximately 1 micron in diameter and are so called because they produce molecular hydrogen. The anaerobic *ciliate protozoan Nyctotherus ovalis, found in the hindgut of several species of cockroach, has numerous hydrogenosomes that are intimately associated with *endosymbiotic methane-producing *Archaea, the latter using the hydrogen produced by the hydrogenosomes.
Like *mitochondria, the hydrogenosomes are bound by distinct double membranes and have an inner membrane with *cristae-like projections. The matrix contains *ribosome-like particles of the same size as a numerous type of ribosome (70s) of the endosymbiotic methanogenic Archaea. It has been postulated that hydrogenosomes evolved from mitochondria by the concomitant loss of their respiration and organellar genomes, and indeed so far no hydrogenosome has been found that has a genome.
The following points are made by A. Akhmanova et al (Nature 1998 396:527):
1) The authors report evidence of a hydrogenosomal genome of apparent mitochondrial descent, and also evidence that the protozoan N. ovalis possesses a new type of nucleus-encoded "iron-only" *hydrogenase.
2) The authors suggest their results indicate that N. ovalis hydrogenosomes evolved from mitochondria, but that contrary to what has been proposed by others, the hydrogenosomes have not relinquished their genomes. The authors further suggest that the evolutionary origin of the N. ovalis nuclear hydrogenase gene remains puzzling.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
anaerobic: Refers to a life form or process sustained in the absence of free (gaseous or dissolved) oxygen.
protozoa: A phylum (or subkingdom) comprising unicellular and colonial animals of varied form, cells ranging from simple to extremely complex macro-structures.
fungi: A kingdom of primarily multicellular organisms lacking chlorophyll and existing as parasites, symbionts, or saprophytes. In general, "symbionts" are organisms that live in close association (symbiosis) with other organisms; saprophytes are organisms that feed on dead or decaying life forms.
ciliate: Cilia are short threadlike extensions, hundreds usually present on an individual ciliated cell, the cilia undergoing synchronized movements to produce locomotion of the protozoan.
endosymbiotic: Endosymbiosis is an arrangement in which one organism lives inside another organism, but the term is usually restricted to arrangements of mutual benefit, thus not including parasite-host relationships. A number of eukaryotic cell organelles (including mitochondria) are believed to have originated from endosymbiotic relationships between eukaryotic cells and simpler cells.
Archaea: The archaebacteria (also called the Archaea) are a subkingdom of bacteria considered to be ancient compared to other bacterial kingdoms, and possibly the most ancient life forms and the ancestors of all eukaryotes (cells and organisms with intracellular membrane-bound organelles). They typically exist in extreme environments, and include the methane-producing bacteria (methanogens), the "salt-loving" bacteria (halophilic bacteria), and the sulfur-acid tolerant thermoacidophilic bacteria.
mitochondria: Mitochondria are double-membrane enclosed organelles of cells that are involved with several important biochemical pathways, including electron transport and oxidative metabolism.
cristae: The folds of the inner mitochondrial membrane. They contain the enzymes and other proteins involved in mitochondrial metabolism.
ribosome: A ribosome (not to be confused with riboZYME) is a small particle, a complex of various ribonucleic acid component subunits and proteins that functions as the site of protein synthesis.
hydrogenase: (hydrogenlyase) An enzyme that catalyzes the interaction of reduced *ferredoxin with ionic hydrogen to produce oxidized ferredoxin and molecular hydrogen.
ferredoxin: In general, a simple non-enzymic iron-sulfur protein. Such proteins act as electron carriers in a variety of oxidation-reduction systems and are found in a wide range of microorganisms, in chloroplasts, and in some types of mitochondria.
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