Transcription translation feedback loop

Last updated

Transcription-translation feedback loop (TTFL) is a cellular model for explaining circadian rhythms in behavior and physiology. Widely conserved across species, the TTFL is auto-regulatory, in which transcription of clock genes is regulated by their own protein products.

Contents

Discovery

Circadian rhythms have been documented for centuries. For example, French astronomer Jean-Jacques d’Ortous de Mairan noted the periodic 24-hour movement of Mimosa plant leaves as early as 1729. However, science has only recently begun to uncover the cellular mechanisms responsible for driving observed circadian rhythms. The cellular basis of circadian rhythms is supported by the fact that rhythms have been observed in single-celled organisms [1]

Beginning in the 1970s, experiments conducted by Ron Konopka and colleagues, in which forward genetic methods were used to induce mutation, revealed that Drosophila melanogaster specimens with altered period (Per) genes also demonstrated altered periodicity. As genetic and molecular biology experimental tools improved, researchers further identified genes involved in sustaining normal rhythmic behavior, giving rise to the concept that internal rhythms are modified by a small subset of core clock genes. Hardin and colleagues (1990) were the first to propose that the mechanism driving these rhythms was a negative feedback loop. Subsequent major discoveries confirmed this model; notably experiments led by Thomas K. Darlington and Nicholas Gekakis in the late 1990s that identified clock proteins and characterized their methods in Drosophila and mice, respectively. These experiments gave rise to the transcription-translation feedback loop (TTFL) model that has now become the dominant paradigm for explaining circadian behavior in a wide array of species. [2]

General mechanisms of TTFL

The TTFL is a negative feedback loop, in which clock genes are regulated by their protein products. Generally, the TTFL involves two main arms: positive regulatory elements that promote transcription and protein products that suppress transcription. When a positive regulatory element binds to a clock gene promoter, transcription proceeds, resulting in the creation of an mRNA transcript, and then translation proceeds, resulting in a protein product. There are characteristic delays between mRNA transcript accumulation, protein accumulation, and gene suppression due to translation dynamics, post-translational protein modification, protein dimerization, and intracellular travel to the nucleus. [3] Across species, proteins involved in the TTFL contain common structural motifs such PAS domains, involved in protein-protein interactions, and bHLH domains, involved in DNA binding. [4]

Once enough modified protein products accumulate in the cytoplasm, they are transported into the nucleus where they inhibit the positive element from the promoter to stop transcription of clock genes. The clock gene is thus transcribed at low levels until its protein products are degraded, allowing for positive regulatory elements to bind to the promoter and restart transcription. The negative feedback loop of the TTFL has multiple properties important for the cellular circadian clock. First, it results in daily rhythms in both gene transcription and protein abundance and size, caused by the delay between translation and negative regulation of the gene. The cycle's period, or time required to complete one cycle, remains consistent in each individual and, barring mutation, is typically near 24 hours. This enables stable entrainment to the 24 hour light-dark cycle that Earth experiences. Additionally, the protein products of clock genes control downstream genes that are not part of the feedback loop, allowing clock genes to create daily rhythms in other processes, such as metabolism, within the organism. [3] Lastly, the TTFL is a limit cycle, meaning that it is a closed loop that will return to its fixed trajectory even if it is disturbed, maintaining the oscillatory path on its fixed 24-hour period. [5]

Prominent models

The presence of the TTFL is highly conserved across animal species; however, many of the players involved in the process have changed across evolutionary time within different species. There are differences in the genes and proteins involved in the TTFL when comparing plants, animals, fungi and other eukaryotes. This suggests that a clock that follows the TTFL model has evolved multiple times during the existence of life. [6]

Regulation of Clock genes
Drosophila melanogaster
Positive RegulatorsCYC, Clock
Negative RegulatorsTIM, PER
Mammals
Positive RegulatorsBMAL1, CLOCK
Negative RegulatorsPER1, PER2, CRY1, CRY2
Neurospora
Positive RegulatorsWC-1. WC-2
Negative RegulatorsFRQ

Drosophila melanogaster

The TTFL was first discovered in Drosophila, and the system shares several components with the mammalian TTFL. Transcription of the clock genes, Period (per) and Timeless (tim), is initiated when positive elements Cycle (dCYC) and Clock (dCLK) form a heterodimer and bind E-box promoters, initiating transcription. During the day TIM is degraded; light exposure facilitates CRY binging to TIM, which leads to TIM's ubiquitination and eventual degradation. [7] During the night, TIM and PER are able to form heterodimers and accumulate slowly in the cytoplasm, where PER is phosphorylated by the kinase DOUBLETIME (DBT). The post-transcriptional modification of multiple phosphate groups both targets the complex for degradation and facilitates nuclear localization. In the nucleus, the PER-TIM dimer binds to the CYC-CLK dimer, which makes the CYC-CLK dimer release from the E-boxes and inhibits transcription. Once PER and TIM degrade, CYC-CLK dimers are able to bind the E-boxes again to initiate transcription, closing the negative feedback loop. [8]

Figure shows the Drosophila melanogaster TTFL and their general interactions between the main players. In this case we can see how CLK and CYC are the positive regulators (yellow and green) and PER and TIM are the negative (red and blue) regulators that each play a role in the circadian clock. Circadian clock of drosophila.PNG
Figure shows the Drosophila melanogaster TTFL and their general interactions between the main players. In this case we can see how CLK and CYC are the positive regulators (yellow and green) and PER and TIM are the negative (red and blue) regulators that each play a role in the circadian clock.

Secondary feedback loops interact with this primary feedback loop. CLOCKWORK ORANGE (CWO) binds the E-boxes to act as a direct competitor of CYC-CLK, therefore inhibiting transcription. PAR-DOMAIN PROTEIN 1 ε (PDP1ε) is a feedback activator and VRILLE (VRI) is a feedback inhibitor of the Clk promoter, and their expression is activated by dCLK-dCYC. Ecdysone-induced protein 75 (E75) inhibits clk expression and tim-dependently activates per transcription. All of these secondary loops act to reinforce the primary TTFL. [8]

Cryptochrome in Drosophila is a blue-light photoreceptor that triggers degradation of TIM, indirectly leading to the clock phase being reset and the renewed promotion of per expression. [8]

Mammals

Figure shows the mammalian TTFL and the general interactions between the main players. This shows how PER and CRY both are negative regulators (red arrows) for BMAL1 and CLOCK, since they cause inhibition of BMAL1 and CLOCK by preventing transcription. BMAL1 and CLOCK (green arrows) are positive regulators since they encourage the transcription, and later the translation of PER and CRY. Circadian clock of mammals.PNG
Figure shows the mammalian TTFL and the general interactions between the main players. This shows how PER and CRY both are negative regulators (red arrows) for BMAL1 and CLOCK, since they cause inhibition of BMAL1 and CLOCK by preventing transcription. BMAL1 and CLOCK (green arrows) are positive regulators since they encourage the transcription, and later the translation of PER and CRY.

The mammalian TTFL model contains many components that are homologs of the ones found in Drosophila. The way the mammalian system works is that BMAL1 forms a heterodimer with CLOCK to initiate transcription of mPer and cryptochrome (cry). There are three paralogs, or historically similar genes that now appear as a duplication, of the period gene in mammals listed as mPer1, mPer2, and mPer3. There are also two paralogs of cryptochrome in mammals. PER and CRY proteins form a heterodimer, and PER's phosphorylation by CK1δ and CK1ε regulates the localization of the dimer to the nucleus. In the nucleus, PER-CRY negatively regulates the transcription of their cognate genes by binding BMAL1-CLOCK and causing their release from the E-box promoter. [8]

Although the mPer paralogs work together as a functional ortholog of dPer, they each have a distinguished function. mPer1 and mPer2 are necessary for clock function in the brain, while mPer3 only plays a discernible role in the circadian rhythms of peripheral tissues. Knocking out either mPer1 or mPer2 causes a change in period, with mPer1 knockouts free-running with a shorter period and mPer2 knockouts free running with a longer period compared to the original tau before eventually becoming arrhythmic. Similarly, mCry1 knockouts result in a shortened period and mCry2 knockouts result in a lengthened period, with a double mCry1/mCry2 knockouts result in arrhythmicity. [8]

There are also secondary loops in mammals, although they are more complex than those seen in Drosophila. Like CWO in Drosophila, Deleted in esophageal cancer1,2 (Dec1 Dec2) repress mPer expression by binding E-boxes which prevents CLOCK-BMAL1 from binding their targets. The receptors REV-ERB and retinoic acid-related orphan receptor (ROR) play a similar role to PDP1ε and VRI in Drosophila, except they regulate CLOCK's binding partner BMAL1 instead of directly regulating CLOCK. D site-binding protein (DBP) and E4-binding protein (E4BP4) bind to the D-Box promoter sequence to regulate mPer expression. [8]

The way these genes relate to Drosophila melanogaster is seen in the function of each of the genes and how they have evolutionarily changed. BMAL1 is an ortholog of CYCLE. This means that BMAL1 and CYCLE appear to have a common history, but are found in different species. Another example of the parallels between Drosophila melanogaster and mammals is also seen in cry and mPer since they are functional orthologs of per and tim. [8]

Fungi: Neurospora

Overview of the Neurospora TTFL and the general interactions between the regulators. In this case WC-1 and WC-2 (red) are seen as the positive elements where they come together to encourage transcription of FRQ. FRQ (green) is the negative regulator which after translation, comes back as negative feedback. Neurospora2.PNG
Overview of the Neurospora TTFL and the general interactions between the regulators. In this case WC-1 and WC-2 (red) are seen as the positive elements where they come together to encourage transcription of FRQ. FRQ (green) is the negative regulator which after translation, comes back as negative feedback.

The gene frequency (frq) in Neurospora was identified as the second known clock gene in 1979 by JF Feldman and his colleagues. Frq was first cloned in 1989 by CR McClung and his colleagues. This gene was of particular interest because its expression is very complex compared to other known microbial genes. Two positive regulator proteins, White Collar-1 (WC-1) and White Collar-2 (WC-2) bind the frq promoter, which is called the Clock Box, during late subjective night to activate transcription. Light is also important for inducing FRQ expression; WC-1 is a photopigment, and light allows WC-1 and WC-2 to bind another promoter called the proximal light-response element (PLRE). FRQ protein negatively regulates the activity of WC-1 and WC-2. Several kinases (CK1, CK2, and PRD-4/checkpoint kinase 2) and phosphatases (PP1 and PP2A) regulate the ability of FRQ to translocate to the nucleus and FRQ, WC-1 and WC-2 stability. [9]

Plants: Arabidopsis thaliana

The first TTFL model was proposed for Arabidopsis thaliana in 2001 and included two MYB transcription factors, LATE ELONGATED HYPOCOTYL (LHY), CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), and TIMING OF CAB EXPRESSION 1 (TOC1). CCA1 and LHY are expressed in the morning, and interact together to repress the expression of TOC1. CCA1 and LHY expression decreases in the darkness, allowing for TOC1 to express and negatively regulate CCA1 and LHY expression. CCA1 and LHY can also bind to their own promoter to repress their own transcription. [10]

Figure shows the TTFL of plants (Arabidopsis). This shows how the different regulators function and how this still qualifies to be a TTFL because of the feedback loops that occur. Arabidopsis thaliana circadian.PNG
Figure shows the TTFL of plants (Arabidopsis). This shows how the different regulators function and how this still qualifies to be a TTFL because of the feedback loops that occur.

A second loop exists involving PRR9, PRR7, and PRR5, which are all homologs of TOC1 and repress CCA1 and LHY expression. These PRR genes are directly repressed by LHY and TOC1. These genes are also regulated by the “evening complex” (EC), which is formed by LUX ARRHYTHMO (LUX), EARLY FLOWERING 3 (ELF3) and EARLY FLOWERING 4 (ELF4). LUX is a transcription factor with a similar function to MYB, while ELF3 and ELF4 are nuclear proteins whose functions are unknown. The "evening complex" indirectly promotes the expression of LHY and CCA1, which repress transcription of its own components. Since this model consists of two inhibitions leading to an activation, it is also referred to as a repressilator. [10]

A recently discovered loop includes the reveille (reveille) family of genes, which are expressed in the morning and induce transcription of evening genes such as PRR5, TOC1, LUX, and ELF4. Once the resulting proteins are translated, PRR9, PRR7, and PRR5 repress RVE8. RVE8 also interacts with the NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED (LNK1, 2, 3, and 4) morning components, with LNKs either antagonizing or co-activating RVE8. [10]

Although GIGANTEA (GI) is not known as a core part of the Arabdopsis TTFL model, it is repressed by CCA1, LHY and TOC1. Additionally, GI activates CCA1 and LHY expression. [10]

Cyanobacteria

Studies of the cyanobacteria clock led to the discovery of three essential clock genes: KaiA, KaiB, and KaiC. Initially, these proteins were thought to follow the TTFL model similar to that proposed for eukarya, as there was a daily pattern in mRNA and protein abundance and level of phosphorylation, negative feedback of proteins on their cognate genes, resetting of clock phase in response to KaiC over-expression, and modified Kai activity through interactions with one another. [11] Each of these results was consistent with understandings of the TTFL at the time. However, later studies have since concluded that post translational modifications such as phosphorylation are more important for clock control. When promoters for the Kai proteins were replaced with non-specific promoters, there was no interruption of the central feedback loop, as would be expected if inhibition occurred through the proteins’ feedback onto their specific promoters. As a consequence, the TTFL model has largely been determined to be inaccurate for cyanobacteria; transcriptional regulation is not the central process driving cyanobacteria rhythms. Though transcriptional and translational regulation are present, they were deemed to be effects of the clock rather than necessary for clock function. [12]

Alternatives to the TTFL model

Post-translational feedback loops (PTFLs) involved in clock gene regulation have also been uncovered, often working in tandem with the TTFL model. In both mammals and plants, post-translational modifications such as phosphorylation and acetylation regulate the abundance and/or activity of clock genes and proteins. For example, levels of phosphorylation of TTFL components have been shown to vary rhythmically. These post-translational modifications can serve as degradation signals, binding regulators, and signals for the recruitment of additional factors. [13]

Notably, cyanobacteria demonstrate rhythmic 24-hour changes in phosphorylation in a feedback loop that is independent of transcription and translation: circadian rhythms in phosphorylation are observed when the feedback loop Kai proteins are placed in a test tube with ATP, independent of any other cellular machinery. This three-protein post-translational system is widely accepted to be the core oscillator, both necessary and sufficient to drive daily rhythms. [14] In addition to the Kai system in cyanobacteria, oxidation of peroxiredoxin proteins has been shown to occur independently of transcription and translation in both mammalian red blood cells and algae Ostreococcus tauri cells; this system has been seen to be conserved in many organisms. [15] It is not clear whether the peroxiredoxin system interacts with TTFL-based clocks or is itself a part of a new PTFL-based clock. However, both of these findings imply that in some organisms or cell types, PTFLs are sufficient to drive circadian rhythms.

See also

Related Research Articles

A circadian clock, or circadian oscillator, also known as one’s internal alarm clock is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time.

<span class="mw-page-title-main">CLOCK</span> Human protein and coding gene

CLOCK is a gene encoding a basic helix-loop-helix-PAS transcription factor that is known to affect both the persistence and period of circadian rhythms.

<span class="mw-page-title-main">NPAS2</span> Protein-coding gene in the species Homo sapiens

Neuronal PAS domain protein 2 (NPAS2) also known as member of PAS protein 4 (MOP4) is a transcription factor protein that in humans is encoded by the NPAS2 gene. NPAS2 is paralogous to CLOCK, and both are key proteins involved in the maintenance of circadian rhythms in mammals. In the brain, NPAS2 functions as a generator and maintainer of mammalian circadian rhythms. More specifically, NPAS2 is an activator of transcription and translation of core clock and clock-controlled genes through its role in a negative feedback loop in the suprachiasmatic nucleus (SCN), the brain region responsible for the control of circadian rhythms.

<span class="mw-page-title-main">PER2</span> Protein found in mammals

PER2 is a protein in mammals encoded by the PER2 gene. PER2 is noted for its major role in circadian rhythms.

<span class="mw-page-title-main">Basic helix-loop-helix ARNT-like protein 1</span> Human protein and coding gene

Basic helix-loop-helix ARNT-like protein 1 or aryl hydrocarbon receptor nuclear translocator-like protein 1 (ARNTL), or brain and muscle ARNT-like 1 is a protein that in humans is encoded by the BMAL1 gene on chromosome 11, region p15.3. It's also known as MOP3, and, less commonly, bHLHe5, BMAL, BMAL1C, JAP3, PASD3, and TIC.

In molecular biology, an oscillating gene is a gene that is expressed in a rhythmic pattern or in periodic cycles. Oscillating genes are usually circadian and can be identified by periodic changes in the state of an organism. Circadian rhythms, controlled by oscillating genes, have a period of approximately 24 hours. For example, plant leaves opening and closing at different times of the day or the sleep-wake schedule of animals can all include circadian rhythms. Other periods are also possible, such as 29.5 days resulting from circalunar rhythms or 12.4 hours resulting from circatidal rhythms. Oscillating genes include both core clock component genes and output genes. A core clock component gene is a gene necessary for to the pacemaker. However, an output oscillating gene, such as the AVP gene, is rhythmic but not necessary to the pacemaker.

<i>Cycle</i> (gene)

Cycle (cyc) is a gene in Drosophila melanogaster that encodes the CYCLE protein (CYC). The Cycle gene (cyc) is expressed in a variety of cell types in a circadian manner. It is involved in controlling both the sleep-wake cycle and circadian regulation of gene expression by promoting transcription in a negative feedback mechanism. The cyc gene is located on the left arm of chromosome 3 and codes for a transcription factor containing a basic helix–loop–helix (bHLH) domain and a PAS domain. The 2.17 kb cyc gene is divided into 5 coding exons totaling 1,625 base pairs which code for 413 aminos acid residues. Currently 19 alleles are known for cyc. Orthologs performing the same function in other species include basic helix-loop-helix ARNT-like protein 1 (ARNTL) and Aryl hydrocarbon receptor nuclear translocator-like 2 (ARNTL2).

The frequency (frq) gene encodes the protein frequency (FRQ) that functions in the Neurospora crassa circadian clock. The FRQ protein plays a key role in circadian oscillator, serving to nucleate the negative element complex in the auto regulatory transcription-translation negative feedback-loop (TTFL) that is responsible for circadian rhythms in N. crassa. Similar rhythms are found in mammals, Drosophila and cyanobacteria. Recently, FRQ homologs have been identified in several other species of fungi. Expression of frq is controlled by the two transcription factors white collar-1 (WC-1) and white collar-2 (WC-2) that act together as the White Collar Complex (WCC) and serve as the positive element in the TTFL. Expression of frq can also be induced through light exposure in a WCC dependent manner. Forward genetics has generated many alleles of frq resulting in strains whose circadian clocks vary in period length.

Doubletime (DBT), also known as discs overgrown (DCO), is a gene that encodes the doubletime protein in fruit flies. Michael Young and his team at Rockefeller University first identified and characterized the gene in 1998.

Timing of CAB expression 1 is a protein that in Arabidopsis thaliana is encoded by the TOC1 gene. TOC1 is also known as two-component response regulator-like APRR1.

<span class="mw-page-title-main">Michael Rosbash</span> American geneticist and chronobiologist (born 1944)

Michael Morris Rosbash is an American geneticist and chronobiologist. Rosbash is a professor and researcher at Brandeis University and investigator at the Howard Hughes Medical Institute. Rosbash's research group cloned the Drosophila period gene in 1984 and proposed the Transcription Translation Negative Feedback Loop for circadian clocks in 1990. In 1998, they discovered the cycle gene, clock gene, and cryptochrome photoreceptor in Drosophila through the use of forward genetics, by first identifying the phenotype of a mutant and then determining the genetics behind the mutation. Rosbash was elected to the National Academy of Sciences in 2003. Along with Michael W. Young and Jeffrey C. Hall, he was awarded the 2017 Nobel Prize in Physiology or Medicine "for their discoveries of molecular mechanisms controlling the circadian rhythm".

<i>KaiC</i> Gene found in cyanobacteria

KaiC is a gene belonging to the KaiABC gene cluster that, together, regulate bacterial circadian rhythms, specifically in cyanobacteria. KaiC encodes the KaiC protein, which interacts with the KaiA and KaiB proteins in a post-translational oscillator (PTO). The PTO is cyanobacteria master clock that is controlled by sequences of phosphorylation of KaiC protein. Regulation of KaiABC expression and KaiABC phosphorylation is essential for cyanobacteria circadian rhythmicity, and is particularly important for regulating cyanobacteria processes such as nitrogen fixation, photosynthesis, and cell division. Studies have shown similarities to Drosophila, Neurospora, and mammalian clock models in that the kaiABC regulation of the cyanobacteria slave circadian clock is also based on a transcription translation feedback loop (TTFL). KaiC protein has both auto-kinase and auto-phosphatase activity and functions as the circadian regulator in both the PTO and the TTFL. KaiC has been found to not only suppress kaiBC when overexpressed, but also suppress circadian expression of all genes in the cyanobacterial genome.

White Collar-1 (wc-1) is a gene in Neurospora crassa encoding the protein WC-1. WC-1 has two separate roles in the cell. First, it is the primary photoreceptor for Neurospora and the founding member of the class of principle blue light photoreceptors in all of the fungi. Second, it is necessary for regulating circadian rhythms in FRQ. It is a key component of a circadian molecular pathway that regulates many behavioral activities, including conidiation. WC-1 and WC-2, an interacting partner of WC-1, comprise the White Collar Complex (WCC) that is involved in the Neurospora circadian clock. WCC is a complex of nuclear transcription factor proteins, and contains transcriptional activation domains, PAS domains, and zinc finger DNA-binding domains (GATA). WC-1 and WC-2 heterodimerize through their PAS domains to form the White Collar Complex (WCC).

Circadian Clock Associated 1 (CCA1) is a gene that is central to the circadian oscillator of angiosperms. It was first identified in Arabidopsis thaliana in 1993. CCA1 interacts with LHY and TOC1 to form the core of the oscillator system. CCA1 expression peaks at dawn. Loss of CCA1 function leads to a shortened period in the expression of many other genes.

Paul Hardin is an American scientist in the field of chronobiology and a pioneering researcher in the understanding of circadian clocks in flies and mammals. Hardin currently serves as a distinguished professor in the biology department at Texas A&M University. He is best known for his discovery of circadian oscillations in the mRNA of the clock gene Period (per), the importance of the E-Box in per activation, the interlocked feedback loops that control rhythms in activator gene transcription, and the circadian regulation of olfaction in Drosophila melanogaster. Born in a suburb of Chicago, Matteson, Illinois, Hardin currently resides in College Station, Texas, with his wife and three children.

Vrille (vri) is a bZIP transcription factor found on chromosome 2 in Drosophila melanogaster. Vrille mRNA and protein product (VRI) oscillate predictably on a 24-hour timescale and interact with other circadian clock genes to regulate circadian rhythms in Drosophila. It is also a regulator in embryogenesis; it is expressed in multiple cell types during multiple stages in development, coordinating embryonic dorsal/ventral polarity, wing-vein differentiation, and ensuring tracheal integrity. It is also active in the embryonic gut but the precise function there is unknown. Mutations in vri alter circadian period and cause circadian arrhythmicity and developmental defects in Drosophila.

Pseudo-response regulator (PRR) refers to a group of genes that regulate the circadian oscillator in plants. There are four primary PRR proteins that perform the majority of interactions with other proteins within the circadian oscillator, and another (PRR3) that has limited function. These genes are all paralogs of each other, and all repress the transcription of Circadian Clock Associated 1 (CCA1) and Late Elongated Hypocotyl (LHY) at various times throughout the day. The expression of PRR9, PRR7, PRR5 and TOC1/PRR1 peak around morning, mid-day, afternoon and evening, respectively. As a group, these genes are one part of the three-part repressilator system that governs the biological clock in plants.

<i>Drosophila</i> circadian rhythm

Drosophila circadian rhythm is a daily 24-hour cycle of rest and activity in the fruit flies of the genus Drosophila. The biological process was discovered and is best understood in the species Drosophila melanogaster. Many behaviors are under circadian control including eclosion, locomotor activity, feeding, and mating. Locomotor activity is maximum at dawn and dusk, while eclosion is at dawn.

The Late Elongated Hypocotyl gene (LHY), is an oscillating gene found in plants that functions as part of their circadian clock. LHY encodes components of mutually regulatory negative feedback loops with Circadian Clock Associated 1 (CCA1) in which overexpression of either results in dampening of both of their expression. This negative feedback loop affects the rhythmicity of multiple outputs creating a daytime protein complex. LHY was one of the first genes identified in the plant clock, along with TOC1 and CCA1. LHY and CCA1 have similar patterns of expression, which is capable of being induced by light. Single loss-of-function mutants in both genes result in seemingly identical phenotypes, but LHY cannot fully rescue the rhythm when CCA1 is absent, indicating that they may only be partially functionally redundant. Under constant light conditions, CCA1 and LHY double loss-of-function mutants fail to maintain rhythms in clock-controlled RNAs.

Jay Dunlap is an American chronobiologist and photobiologist who has made significant contributions to the field of chronobiology by investigating the underlying mechanisms of circadian systems in Neurospora, a fungus commonly used as a model organism in biology, and in mice and mammalian cell culture models. Major contributions by Jay Dunlap include his work investigating the role of frq and wc clock genes in circadian rhythmicity, and his leadership in coordinating the whole genome knockout collection for Neurospora. He is currently the Nathan Smith Professor of Molecular and Systems Biology at the Geisel School of Medicine at Dartmouth. He and his colleague Jennifer Loros have mentored numerous students and postdoctoral fellows, many of whom presently hold positions at various academic institutions.

References

  1. ↑ Mergenhagen D (2001). "Circadian rhythms in unicellular organisms". Current Topics in Microbiology and Immunology. Handbook of Behavioral Neurobiology. 90. Springer US: 123–47. doi:10.1007/978-1-4615-1201-1_4. ISBN   9781461512011. PMID   6775877.
  2. ↑ Wulund L, Reddy AB (December 2015). "A brief history of circadian time: The emergence of redox oscillations as a novel component of biological rhythms". Perspectives in Science. 6: 27–37. doi: 10.1016/j.pisc.2015.08.002 .
  3. 1 2 Hastings MH, Maywood ES, O'Neill JS (September 2008). "Cellular circadian pacemaking and the role of cytosolic rhythms". Current Biology. 18 (17): R805–R815. doi: 10.1016/j.cub.2008.07.021 . PMID   18786386.
  4. ↑ Dunlap JC, Loros JJ, Liu Y, Crosthwaite SK (January 1999). "Eukaryotic circadian systems: cycles in common". Genes to Cells. 4 (1): 01–10. doi: 10.1046/j.1365-2443.1999.00239.x . PMID   10231388.
  5. ↑ Sheredos B (2013). "Scientific Diagrams as Traces of Group-Dependent Cognition: A Brief Cognitive-Historical Analysis". Proceedings of the Annual Meeting of the Cognitive Science Society. 35 (35).
  6. ↑ Loudon AS (July 2012). "Circadian biology: a 2.5 billion year old clock". Current Biology. 22 (14): R570-1. doi: 10.1016/j.cub.2012.06.023 . PMID   22835791.
  7. ↑ Yoshii T, Hermann-Luibl C, Helfrich-Förster C (2016-01-02). "Circadian light-input pathways in Drosophila". Communicative & Integrative Biology. 9 (1): e1102805. doi:10.1080/19420889.2015.1102805. PMC   4802797 . PMID   27066180.
  8. 1 2 3 4 5 6 7 Andreani TS, Itoh TQ, Yildirim E, Hwangbo DS, Allada R (December 2015). "Genetics of Circadian Rhythms". Sleep Medicine Clinics. 10 (4): 413–21. doi:10.1016/j.jsmc.2015.08.007. PMC   4758938 . PMID   26568119.
  9. ↑ Dunlap JC, Loros JJ, Colot HV, Mehra A, Belden WJ, Shi M, Hong CI, Larrondo LF, Baker CL, Chen CH, Schwerdtfeger C, Collopy PD, Gamsby JJ, Lambreghts R (2007). "A circadian clock in Neurospora: how genes and proteins cooperate to produce a sustained, entrainable, and compensated biological oscillator with a period of about a day". Cold Spring Harbor Symposia on Quantitative Biology. 72: 57–68. doi:10.1101/sqb.2007.72.072. PMC   3683860 . PMID   18522516.
  10. 1 2 3 4 Sanchez SE, Kay SA (December 2016). "The Plant Circadian Clock: From a Simple Timekeeper to a Complex Developmental Manager". Cold Spring Harbor Perspectives in Biology. 8 (12): a027748. doi:10.1101/cshperspect.a027748. PMC   5131769 . PMID   27663772.
  11. ↑ Johnson CH, Mori T, Xu Y (September 2008). "A cyanobacterial circadian clockwork". Current Biology. 18 (17): R816–R825. doi:10.1016/j.cub.2008.07.012. PMC   2585598 . PMID   18786387.
  12. ↑ Sheredos B (2013). "Scientific Diagrams as Traces of Group-Dependent Cognition: A Brief Cognitive-Historical Analysis". Proceedings of the Annual Meeting of the Cognitive Science Society. 35 (35).
  13. ↑ Kojima S, Shingle DL, Green CB (February 2011). "Post-transcriptional control of circadian rhythms". Journal of Cell Science. 124 (Pt 3): 311–20. doi:10.1242/jcs.065771. PMC   3021995 . PMID   21242310.
  14. ↑ Hurley JM, Loros JJ, Dunlap JC (October 2016). "Circadian Oscillators: Around the Transcription-Translation Feedback Loop and on to Output". Trends in Biochemical Sciences. 41 (10): 834–846. doi:10.1016/j.tibs.2016.07.009. PMC   5045794 . PMID   27498225.
  15. ↑ Brown SA, Kowalska E, Dallmann R (March 2012). "(Re)inventing the circadian feedback loop". Developmental Cell. 22 (3): 477–87. doi: 10.1016/j.devcel.2012.02.007 . PMID   22421040.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.