The CoRR hypothesis states that the location of

genetic information A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequences are usu ...

cytoplasmic In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. T ...

organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...

permits regulation of its expression by the reduction-oxidation ("

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

") state of its

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 ...

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 is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. ...

". CoRR was put forward explicitly in 1993 in a paper in the ''

Journal of Theoretical Biology The ''Journal of Theoretical Biology'' is a biweekly peer-reviewed scientific journal covering theoretical biology, as well as mathematical, computational, and statistical aspects of biology. Some research areas covered by the journal include cel ...

'' 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'' is a scientific journal published by the Royal Society. In its earliest days, it was a private venture of the Royal Society's secretary. It was established in 1665, making it the first journa ...

'' entitled "The function of genomes in bioenergetic organelles".

The problem

Chloroplasts and mitochondria

Chloroplasts A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...

and mitochondria are energy-converting organelles in the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

eukaryotic cells Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

. Chloroplasts in

plant cells Plant cells are the cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capabi ...

perform

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

; the capture and conversion of the energy of sunlight. Mitochondria in both plant and

animal cells The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, and contains many biomolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients and ...

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 Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respondi ...

. 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 In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gen ...

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 A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

in the nuclei of eukaryotic cells. There they code for protein precursors that are made in the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

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 DNA, and thus why some characters are inherited through the cytoplasm in the phenomenon of cytoplasmic, non-

Mendelian Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later populari ...

, uniparental, or maternal inheritance. CoRR does so by offering an answer to this question: why, in

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

, did some bacterial,

endosymbiont An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "withi ...

genes move to the cell nucleus, while others did not?

Proposed solution

CoRR states that chloroplasts and mitochondria contain those

genes In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

whose expression is required to be under the direct, regulatory control of the redox state of their gene products, or of

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...

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 is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...

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 gene product.

Evidence

* Different products of

protein synthesis Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critical ...

in isolated chloroplasts and mitochondria are obtained in the presence of redox reagents with different

redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

s. In mitochondria, the effect results from a redox signal at the level of respiratory

complex II Succinate dehydrogenase (SDH) or succinate-coenzyme Q reductase (SQR) or respiratory complex II is an enzyme complex, found in many bacterial cells and in the inner mitochondrial membrane of eukaryotes. It is the only enzyme that participates in ...

. * Genes in chloroplasts are selected for transcription according to the redox state of the electron carrier plastoquinone. These genes code for photosynthetic reaction centers and other components of the photosynthetic

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couple ...

. * A modified bacterial sensor

kinase In biochemistry, a kinase () is an enzyme that catalysis, catalyzes the transfer of phosphate groups from High-energy phosphate, high-energy, phosphate-donating molecules to specific Substrate (biochemistry), substrates. This process is known as ...

couples transcription in chloroplasts to plastoquinone redox state. This chloroplast sensor kinase is inherited from cyanobacteria and encoded in plant

nuclear DNA Nuclear DNA (nDNA), or nuclear deoxyribonucleic acid, is the DNA contained within each cell nucleus of a eukaryotic organism. It encodes for the majority of the genome in eukaryotes, with mitochondrial DNA and plastid DNA coding for the rest. ...

Chloroplast sensor kinase Chloroplast Sensor Kinase (CSK) is a protein in chloroplasts and cyanobacteria, bacteria from which chloroplasts evolved by endosymbiosis. It is part of a two-component system. In the plant ''Arabidopsis thaliana'' (thale cress) CSK is the product ...

is part of a redox two-component regulatory system in eukaryotes. * Products of genes most commonly retained in mitochondrial DNA fulfil central roles in the structure of their respective protein complexes, suggesting that their organellar retention allows local control of complex assembly.

References

{{Use dmy dates, date=April 2017 Bacteria Cell biology Eukaryote biology Evolutionary biology Genomics Mitochondrial genetics Organelles