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The CoRR Hypothesis for Organelle Gene Retention

The CoRR hypothesis, or 'co-location for redox regulation,' explains why certain genes are retained in eukaryotic cell organelles like mitochondria and chloroplasts. It suggests that the proximity of these genes to their gene products is crucial for regulating gene expression in response to the redox state. This concept supports the endosymbiotic theory and provides insight into the evolutionary conservation of organelle genomes, highlighting the importance of redox regulation in cellular energy dynamics.

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1

CoRR hypothesis origin timeframe

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Proposed in the 1990s

2

CoRR hypothesis on gene retention rationale

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Explains retention of certain genes in organelles for redox regulation efficiency

3

CoRR hypothesis vs. nuclear genome centralization

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Addresses why distinct genetic systems in organelles persist despite most genetic info in nucleus

4

Chloroplasts are found in ______ and certain ______, performing ______.

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plants protists photosynthesis

5

Mitochondria, known as the ______ of the cell, are present in all ______ and are responsible for ______.

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powerhouses eukaryotes cellular respiration

6

Both chloroplasts and mitochondria contain their own ______, which is evidence for the ______ theory.

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DNA endosymbiotic

7

The ______ theory posits that chloroplasts and mitochondria were once ______ prokaryotes.

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endosymbiotic free-living

8

Mitochondria and chloroplasts: separate genomes significance?

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Indicates endosymbiotic origin; organelles were once independent prokaryotes.

9

Metabolic cost of nuclear genome duplicating organelle-specific genes?

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High energy expenditure for synthesis of organelle proteins; suggests evolutionary advantage.

10

Gene proximity to organelle's biochemical environment: why crucial?

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Enables immediate, precise gene expression regulation; influenced by organelle's redox state.

11

Genes crucial for ______ and ______ are kept in organelles due to natural selection for bioenergetic efficiency.

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photosynthesis respiration

12

A secondary group of genes, not involved in redox control but essential for gene expression, is also preserved within ______.

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organelles

13

The hypothesis foresees that transferring key genes to the ______ may lead to organelles losing their genomes over time.

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nucleus

14

Effect of plastoquinone redox state on gene transcription

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In chloroplasts, plastoquinone redox state influences transcription of photosynthesis-related genes.

15

Role of sensor kinase in chloroplasts

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Sensor kinase, encoded by nuclear DNA, regulates chloroplast gene expression based on redox state.

16

Significance of mitochondrial DNA gene retention

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Mitochondrial DNA retains genes for electron transport chain components, enabling local control of protein complex assembly/function.

17

The importance of ______ regulation in the evolution of eukaryotic cells is underscored by the CoRR hypothesis.

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redox

18

According to the CoRR hypothesis, there are functional advantages to keeping genes within ______.

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organelles

19

Our understanding of cellular biology and the evolutionary forces shaping life is enhanced by insights into ______ genome conservation.

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organelle

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Exploring the CoRR Hypothesis for Organelle Gene Retention

The CoRR hypothesis, short for "co-location for redox regulation," provides an evolutionary rationale for why certain genes are retained within the organelles of eukaryotic cells, such as mitochondria and chloroplasts, rather than being transferred to the nucleus. This theory posits that the physical proximity of these genes to their gene products is essential for the efficient regulation of gene expression in response to the redox state, which is the balance of reduction and oxidation reactions. Proposed in the 1990s, the CoRR hypothesis has been refined to explain the selective advantage of maintaining distinct genetic systems within organelles, despite the centralization of most genetic information within the nuclear genome.
Close-up of a green leaf with visible vascular system and representations of a semi-transparent mitochondrion and chloroplast on the blurred natural background.

The Essential Functions of Chloroplasts and Mitochondria

Chloroplasts and mitochondria are vital organelles in eukaryotic cells, central to energy metabolism. Chloroplasts, exclusive to plants and some protists, are the sites of photosynthesis, where light energy is converted into chemical energy. Mitochondria, ubiquitous in eukaryotes, are the powerhouses of the cell, orchestrating cellular respiration to generate ATP from organic molecules. Both organelles harbor their own DNA, which encodes some of the proteins required for their functions. This genetic autonomy supports the endosymbiotic theory, which suggests that these organelles originated from once free-living prokaryotes that entered into a symbiotic relationship with a host cell.

The Evolutionary Enigma of Organelle Genomes

The existence of separate genomes in mitochondria and chloroplasts, while other organelles lack their own DNA, presents an evolutionary puzzle. The duplication of genetic material and the synthesis of organelle-specific proteins by the nuclear genome represent a significant metabolic cost. The CoRR hypothesis addresses this puzzle by suggesting that the proximity of certain genes to the biochemical environment of the organelle is necessary for the immediate and precise regulation of their expression, which is influenced by the redox state of the organelle's internal milieu.

Redox Control and Gene Expression in Organelles

The CoRR hypothesis identifies a primary subset of genes within organelles that require direct redox control for proper expression. These genes are often involved in energy conversion processes, such as electron transport in photosynthesis and respiration. Natural selection has favored the retention of these genes within organelles to ensure efficient bioenergetic function. A secondary subset of genes, related to the organelles' own genetic machinery, is also retained. Although not directly involved in redox regulation, these genes are necessary for the expression of the primary subset. The hypothesis predicts that if these primary subset genes were transferred to the nucleus, the organelles might eventually lose their genomes due to the lack of selective pressure to maintain them.

Empirical Support for the CoRR Hypothesis

Experimental data underpin the CoRR hypothesis. Research has demonstrated that the protein synthesis profiles of isolated chloroplasts and mitochondria change under varying redox conditions, indicating a redox-dependent regulation of gene expression. In chloroplasts, the redox state of plastoquinone, an electron carrier, affects the transcription of genes involved in photosynthesis. Additionally, a sensor kinase, originating from the organelles' bacterial ancestors and now encoded in the nuclear genome, modulates gene expression in chloroplasts in response to their redox state. This kinase is part of a conserved two-component regulatory system. Genes retained in mitochondrial DNA often encode components of the electron transport chain, suggesting that their location within the mitochondria allows for localized control over the assembly and function of these critical protein complexes.

The CoRR Hypothesis: Implications and Significance

The CoRR hypothesis offers a persuasive explanation for the evolutionary conservation of organelle genomes, shedding light on the relationship between gene location, expression, and cellular energy dynamics. It highlights the significance of redox regulation in the evolutionary history of eukaryotic cells and the functional benefits of maintaining genes within organelles. This understanding not only advances our grasp of cellular biology but also informs our broader appreciation of the evolutionary forces that have sculpted the intricate organization of life.