The
CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation ("
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
") state of its
gene productA gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be correlated with disease-causing alleles, such as the...
s.
CoRR is short for "
Co-location for
Redox
Regulation", itself a shortened form of "
Co-location (of gene and gene product) for (evolutionary) continuity of
Redox
Regulation of
gene expressionGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA...
."
CoRR was put forward explicitly in 1993 in a paper in the
Journal of Theoretical BiologyThe Journal of Theoretical Biology is a scientific journal about theoretical biology; dealing with theoretical issues, as well as mathematical and computational aspects of biology. Some research areas covered by the papers published in the journal are population genetics, morphogenesis,...
with the title
Control of gene expression by redox potential and the requirement for chloroplast and mitochondrial genomes .
The
CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation ("
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
") state of its
gene productA gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be correlated with disease-causing alleles, such as the...
s.
CoRR is short for "
Co-location for
Redox
Regulation", itself a shortened form of "
Co-location (of gene and gene product) for (evolutionary) continuity of
Redox
Regulation of
gene expressionGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA...
."
CoRR was put forward explicitly in 1993 in a paper in the
Journal of Theoretical BiologyThe Journal of Theoretical Biology is a scientific journal about theoretical biology; dealing with theoretical issues, as well as mathematical and computational aspects of biology. Some research areas covered by the papers published in the journal are population genetics, morphogenesis,...
with the title
Control of gene expression by redox potential and the requirement for chloroplast and mitochondrial genomes . The central concept had been outlined in a review of 1992 . The term
CoRR was introduced in 2003 in a paper in
Philosophical Transactions of the Royal SocietyThe Philosophical Transactions of the Royal Society, or Phil. Trans., is a scientific journal published by the Royal Society.Begun in 1665, it is the oldest scientific journal printed in the English-speaking world and the second oldest in the world, after the French Journal des sçavans...
entitled
The function of genomes in bioenergetic organelles .
The problem
Chloroplasts and mitochondria.
Chloroplasts and mitochondria are energy-converting organelles in the
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
of eukaryotic cells. Chloroplasts in plant cells perform
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
; the capture and conversion of the
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
of
sunlightSunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. Near the poles in summer, the days are longer and the...
. Mitochondria in both plant and animal cells perform
respirationCellular respiration is one of the key ways a cell gains useful energy. It is the set of the metabolic reactions and processes that take place in organisms' cells to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products...
; the release of this stored energy when work is done. In addition to these key reactions of
bioenergeticsBioenergetics is the subject of a field of biochemistry that concerns energy flow through living systems. This is an active area of biological research that includes the study of thousands of different cellular processes such as cellular respiration and the many other metabolic processes that can...
, chloroplasts and mitochondria each contain specialized and discrete genetic systems. These genetic systems enable chloroplasts and mitochondria to make some of their own proteins. Both the genetic and energy-converting systems of chloroplasts and mitochondria are descended, with little modification, from those of the free-living
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that these organelles once were. The existence of these cytoplasmic genomes is consistent with, and counts as evidence for, the endosymbiont hypothesis. Most genes for proteins of chloroplasts and mitochondria are, however, now located on chromosomes in the nuclei of eukaryotic cells. There they code for protein precursors that are made in the
cytosolThe cytosol or intracellular fluid is the liquid found inside cells. In eukaryotes this liquid is separated by cell membranes from the contents of the organelles suspended in the cytosol, such as the mitochondrial matrix inside the mitochondrion...
for subsequent import into the organelles.
Why do mitochondria and chloroplasts have their own genetic systems?
Why do mitochondria and chloroplasts require their own separate genetic systems, when other organelles that share the same cytoplasm, such as peroxisomes and lysosomes, do not? The question is not trivial, because maintaining a separate genetic system is costly: more than 90 proteins ... must be encoded by nuclear genes specifically for this purpose. ... The reason for such a costly arrangement is not clear, and the hope that the nucleotide sequences of mitochondrial and chloroplast genomes would provide the answer has proved to be unfounded. We cannot think of compelling reasons why the proteins made in mitochondria and chloroplasts should be made there rather than in the cytosol.
Quoted from Alberts et al. The Molecular Biology of the Cell. Garland Science. All editions including 4th Edition on-line.
Cytoplasmic inheritance.
CoRR seeks to explain why chloroplasts and mitochondria retain
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
, and thus why some characters are inherited through the
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
in the phenomenon of cytoplasmic, non-Mendelian, uniparental, or maternal inheritance. CoRR does so by offering an answer to this question; why, in
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
, did some bacterial,
endosymbiontAn endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
genes move to the
cell nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
, while others did not?
Proposed solution
CoRR states that chloroplasts and mitochondria contain those
genesGênes is the name of a département of the First French Empire in present Italy. It was named after the city Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa. It was divided into the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
whose expression is required to be under the direct, regulatory control of the
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
state of their gene products, or of
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
carriers with which those gene products interact. Such genes comprise a core, or primary subset, of organellar genes. The requirement for redox control of each gene in the primary subset then confers an advantage upon location of that gene within the organelle.
Natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
therefore anchors some genes in organelles, while favouring location of others in the cell nucleus. Chloroplast and mitochondrial genomes also contain genes for components of the chloroplast and mitochondrial genetic systems themselves. These genes comprise a secondary subset of organellar genes; genetic system genes. There is generally no requirement for redox control of expression of genetic system genes, though their being subject to redox control may, in some cases, allow amplification of
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
signals acting upon
genesGênes is the name of a département of the First French Empire in present Italy. It was named after the city Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa. It was divided into the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
in the primary subset (bioenergetic genes). Retention of genes of the secondary subset (genetic system genes) is necessary for the operation of redox control of expression of genes in the primary subset. If all genes disappear from the primary subset,
CoRR predicts that there is no function for genes in the secondary subset, and such organelles will then, eventually, lose their genomes completely. However, if even only one gene remains under redox control, then an organelle genetic system is required for the synthesis of its single gene product.
Evidence
- Different products of protein synthesis in isolated chloroplasts and mitochondria are obtained in the presence of redox reagents with different redox potentials . In mitochondria, the effect results from a redox signal at the level of respiratory complex II .
- Genes in chloroplasts are selected for transcription
Transcription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes...
according to the redox state of the electron carrier plastoquinonePlastoquinone is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced , forming plastoquinol...
. These genes code for photosynthetic reaction centers and other components of the photosynthetic electron transport chainAn electron transport chain couples a chemical reaction between an electron donor and an electron acceptor to the transfer of H+ ions across a membrane, through a set of mediating biochemical reactions...
.
- A modified bacterial sensor kinase
In chemistry and biochemistry, a kinase, alternatively known as a phosphotransferase, is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates. The process is referred to as phosphorylation...
couples transcriptionTranscription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes...
in chloroplasts to plastoquinonePlastoquinone is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced , forming plastoquinol...
redox state . This chloroplast sensor kinase is inherited from cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
and encoded in plant nuclear DNANuclear DNA, nuclear deoxyribonucleic acid , is DNA contained within a nucleus of eukaryotic organisms. In most cases it encodes more of the genome than the mitochondrial DNA and is passed sexually rather than matrilineally. Nuclear DNA is the most common DNA used in forensic examinations....
. Chloroplast sensor kinase is part of a redox two-component regulatory system in eukaryotes .
See also
- Allen JF, Raven JA (1996) Free-radical-induced mutation vs redox regulation: Costs and benefits of genes in organelles. Journal of Molecular Evolution 42: 482-492
- Allen JF (1996) Separate sexes and the mitochondrial theory of ageing. Journal of Theoretical Biology 180: 135-140
- Race HL, Herrmann RG and Martin W (1999) Why have organelles retained genomes? Trends in Genetics 15: 364-370
- Allen JF, Puthiyaveetil S, Strom J, Allen CA (2005) Energy transduction anchors genes in organelles. Bioessays 27: 426-435
Related pages
- Transfer of mitochondrial and chloroplast DNA to the nucleus
As a logical conclusion of the endosymbiotic theory, since modern-day mitochondrial and chloroplast genomes do not contain a full set of housekeeping genes, and lack many that other descendants of their speculative ancestors share, there must have been a loss of genes...
- Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within cells that convert the energy from food into a form that cells can use...
- Cyanobacteria
Cyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
- Plastids