CoRR Hypothesis
Encyclopedia
The CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation ("redox
") state of its gene product
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 expression
."
CoRR was put forward explicitly in 1993 in a paper in the Journal of Theoretical Biology
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 Society
entitled The function of genomes in bioenergetic organelles.
of eukaryotic cells. Chloroplasts in plant cells perform photosynthesis
; the capture and conversion of the energy
of sunlight
. Mitochondria in both plant and animal cells perform respiration
; the release of this stored energy when work is done. In addition to these key reactions of bioenergetics
, 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 bacteria
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 cytosol
for subsequent import into the organelles.
, and thus why some characters are inherited through the cytoplasm
in the phenomenon of cytoplasmic, non-Mendelian, uniparental, or maternal inheritance. CoRR does so by offering an answer to this question; why, in evolution
, did some bacterial, endosymbiont
genes move to the cell nucleus
, while others did not.
whose expression is required to be under the direct, regulatory control of the redox
state of their gene products, or of electron
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 selection
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 redox
signals acting upon genes
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.
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
") state of its gene product
Gene product
A 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 expression
Gene expression
Gene 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 ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
."
CoRR was put forward explicitly in 1993 in a paper in the Journal of Theoretical Biology
Journal of Theoretical Biology
The 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 Society
Philosophical Transactions of the Royal Society
The Philosophical Transactions of the Royal Society is a scientific journal published by the Royal Society of London. It was established in 1665, making it the first journal in the world exclusively devoted to science, and it has remained in continuous publication ever since, making it the world's...
entitled The function of genomes in bioenergetic organelles.
Chloroplasts and mitochondria
Chloroplasts and mitochondria are energy-converting organelles in the cytoplasmCytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
of eukaryotic cells. Chloroplasts in plant cells perform photosynthesis
Photosynthesis
Photosynthesis is a chemical 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. Photosynthetic organisms are called photoautotrophs, since they can...
; the capture and conversion of the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
of sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
. Mitochondria in both plant and animal cells perform respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
; the release of this stored energy when work is done. In addition to these key reactions of bioenergetics
Bioenergetics
Bioenergetics 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 bacteria
Bacteria
Bacteria are a large domain of prokaryotic 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 cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
for subsequent import into the organelles.
Why do mitochondria and chloroplasts have their own genetic systems?
Cytoplasmic inheritance
CoRR seeks to explain why chloroplasts and mitochondria retain DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, and thus why some characters are inherited through the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
in the phenomenon of cytoplasmic, non-Mendelian, uniparental, or maternal inheritance. CoRR does so by offering an answer to this question; why, in evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, did some bacterial, endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
genes move to the cell nucleus
Cell nucleus
In cell biology, the nucleus 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 histones, to form chromosomes. The genes within these...
, while others did not.
Proposed solution
CoRR states that chloroplasts and mitochondria contain those genesGênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
whose expression is required to be under the direct, regulatory control of the redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
state of their gene products, or of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
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 selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
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 redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
signals acting upon genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in 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 transcriptionTranscription (genetics)Transcription is the process of creating a complementary 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 by the action of the correct enzymes...
according to the redox state of the electron carrier plastoquinonePlastoquinonePlastoquinone 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 chainElectron transport chainAn electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
.
- A modified bacterial sensor kinaseKinaseIn chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
couples transcriptionTranscription (genetics)Transcription is the process of creating a complementary 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 by the action of the correct enzymes...
in chloroplasts to plastoquinonePlastoquinonePlastoquinone 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 cyanobacteria and encoded in plant nuclear DNANuclear DNANuclear DNA, nuclear deoxyribonucleic acid , is DNA contained within a nucleus of eukaryotic organisms. In mammals and vertebrates, nuclear DNA encodes more of the genome than the mitochondrial DNA and is composed of information inherited from two parents, one male, and one female, rather than...
. Chloroplast sensor kinase is part of a redox two-component regulatory system in eukaryotes.
See also
- Transfer of mitochondrial and chloroplast DNA to the nucleusTransfer of mitochondrial and chloroplast DNA to the nucleusAs 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 DNAMitochondrial DNAMitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
- Cyanobacteria
- Plastids