Timeless (gene)
Encyclopedia
Timeless is a gene in Drosophila
which encodes TIM, an essential protein that regulates circadian rhythm
s. Timeless mRNA and protein oscillate rhythmically with time as part of a transcription-translation [negative feedback] loop involving the period
gene and its protein.
The PER/TIM heterodimer regulates transcription of period (per) and timeless (tim) genes. First the PER/TIM heterodimers form in the cytoplasm, accumulate, and then translocate to the nucleus. The complex then blocks the positive transcription factor clock (CLK) and cycle (CYC).
As part of the circadian clock timeless is essential for entrainment
to light-dark (LD) cycles. The typical period length of free-running Drosophila is 23.9 hours, requiring adaptations to the 24-hour environmental cycle. Adaptation first begins with exposure to light. This process leads to the rapid degradation of the TIM protein, allowing organisms to entrain at dawn to environmental cycles. In light-dark cycles, TIM protein level decreases rapidly in late night/early morning, followed by the similar but more gradual changes in PER protein level. TIM degradation is independent of per and its protein, and releases PER from the PER/TIM complex. This ends the PER/TIM repression of the CLK/CYC-mediated transcription of per and tim genes, allowing per and tim mRNA to be produced to restart the cycle. In some cell types, the photoreceptor protein cryptochrome
(CRY) physically associates with TIM and helps regulate light-dependent degradation. CRY is activated by blue light, which binds to TIM and tags it for degradation.
This mechanism allows entrainment of flies to environmental light cues. When Drosophila receive light inputs in the early subjective night, light-induced TIM degradation causes a delay in TIM accumulation, which creates a phase delay. When light inputs are received in the late subjective night, a light pulse causes TIM degradation to occur earlier than under normal conditions, leading to a phase advance.
In Drosophila, the negative factors PER/TIM, as well as the positive factors CLK/CYC, are eventually degraded by a casein kinase
-mediated phosphorylation
cycle, allowing fluctuations in gene expression according to environmental cues. These proteins mediate the oscillating expression of the transcription factor VRILLE (VRI), which is required for behavioral rhythmicity, per and tim expression, and accumulation of PDF
(pigment-dispersing factor).
The timeless protein is thought to directly connect the cell cycle with the circadian rhythm in mammals. In this model called a “direct coupling” the two cycles share a key protein whose expression exhibits a circadian pattern.
as an essential function of the circadian clock. mTim is expressed in the suprachiasmatic nucleus
(SCN), but there is no oscillation of its accompanying RNA or protein products in constant conditions. The mammalian TIM protein levels do not shift with light signals, but there is reported interaction with the mammalian period protein PER1
and mammalian cryptochrome
(CRY1 and CRY2). mTim is shown to be necessary for embryonic development in mice, indicating a different gene function than in Drosophila This suggests a divergence between mammalian clocks and the Drosophila
clock.
(SCN), the major clock governing all tissue-specific circadian rhythms of the body. This protein also interacts with the products of major clock genes CLOCK
, BMAL
, PER1
, PER2
and PER3
.
Sancar and colleagues investigated whether hTIM played a similar role to orthologs in C. elegans
and other types of yeast, which are known to play play important roles in the cell cycle
. Their experiments suggested that hTIM plays an integral role in the G2/M
and intra-S cell cycle checkpoints. With respect to the G2/M checkpoint, hTIM binds to the ATRIP subunit on ATR
– a protein kinase sensitive to DNA damage. This binding between hTIM and ATR then leads to the phosphorylation of Chk1, resulting in cell cycle arrest or apoptosis. This process serves as an important control to stop the proliferation of cells with DNA damage prior to mitotic
division. The role of hTIM in the intra-S checkpoint is less clear at the molecular level. However, down-regulation of hTIM leads to an increase in the rate of generation of replication forks – even in the presence of DNA damage and other regulatory responses.
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
which encodes TIM, an essential protein that regulates circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...
s. Timeless mRNA and protein oscillate rhythmically with time as part of a transcription-translation [negative feedback] loop involving the period
Period (gene)
Period is a gene located on the X chromosome of Drosophila melanogaster. Oscillations in levels of both per transcript and its corresponding protein PER have a period of approximately 24 hours and together play a central role in the molecular mechanism of the Drosophila biological clock driving...
gene and its protein.
Discovery
Timeless was discovered by Dr. Michael W. Young and colleagues in 1994. This gene was found when they noticed an arrhythmic tim01 mutant via a hybrid dysgenesis screen.Function in the circadian clock
The timeless gene is an essential component of the molecular circadian clock in Drosophila. It acts as part of an autoregulatory feedback loop in conjunction with the period (per) gene product. Its circadian properties were noted in studies performed by Dr. Michael Rosbash's lab and Dr. Charles Weitz's lab. Both indicated that timeless protein (TIM) and period protein (PER) form a heterodimer that exhibits circadian rhythms in wild type Drosophila. Researchers in Rosbash's lab also showed that tim mRNA levels and TIM protein levels have circadian rhythms that are similar to those of the period(per) and its product.The PER/TIM heterodimer regulates transcription of period (per) and timeless (tim) genes. First the PER/TIM heterodimers form in the cytoplasm, accumulate, and then translocate to the nucleus. The complex then blocks the positive transcription factor clock (CLK) and cycle (CYC).
As part of the circadian clock timeless is essential for entrainment
Entrainment (chronobiology)
Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. A common example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by...
to light-dark (LD) cycles. The typical period length of free-running Drosophila is 23.9 hours, requiring adaptations to the 24-hour environmental cycle. Adaptation first begins with exposure to light. This process leads to the rapid degradation of the TIM protein, allowing organisms to entrain at dawn to environmental cycles. In light-dark cycles, TIM protein level decreases rapidly in late night/early morning, followed by the similar but more gradual changes in PER protein level. TIM degradation is independent of per and its protein, and releases PER from the PER/TIM complex. This ends the PER/TIM repression of the CLK/CYC-mediated transcription of per and tim genes, allowing per and tim mRNA to be produced to restart the cycle. In some cell types, the photoreceptor protein cryptochrome
Cryptochrome
Cryptochromes are a class of blue light-sensitive flavoproteins found in plants and animals. Cryptochromes are involved in the circadian rhythms of plants and animals, and in the sensing of magnetic fields in a number of species...
(CRY) physically associates with TIM and helps regulate light-dependent degradation. CRY is activated by blue light, which binds to TIM and tags it for degradation.
This mechanism allows entrainment of flies to environmental light cues. When Drosophila receive light inputs in the early subjective night, light-induced TIM degradation causes a delay in TIM accumulation, which creates a phase delay. When light inputs are received in the late subjective night, a light pulse causes TIM degradation to occur earlier than under normal conditions, leading to a phase advance.
In Drosophila, the negative factors PER/TIM, as well as the positive factors CLK/CYC, are eventually degraded by a casein kinase
Casein kinase
Casein kinase is a kinase enzyme.* Casein kinase 1* Casein kinase 2...
-mediated phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
cycle, allowing fluctuations in gene expression according to environmental cues. These proteins mediate the oscillating expression of the transcription factor VRILLE (VRI), which is required for behavioral rhythmicity, per and tim expression, and accumulation of PDF
Pigment dispersing factor
Pigment dispersing factor is a gene that encodes for the protein PDF, which is part of a large family of neuropeptides. The analogous hormone, pigment dispersing hormone was named for the diurnal pigment movement effect it has in crustacean retinal cells, and was initially discovered in the...
(pigment-dispersing factor).
Timeless in the cricket Gryllus bimaculatus
Timeless does not appear to be essential for oscillation of the circadian clock for all insects. In wild type crickets, tim mRNA shows rhythmic expression in both LD and DD (dark-dark cycles) similar to that of per, peaking during the (subjective) night. When injected with tim double-stranded RNA (dstim), tim mRNA levels were significantly reduced and its circadian expression rhythm was eliminated. After the dstim treatment, however, adult crickets showed a clear locomotor rhythm in constant darkness, with a free-running period significantly shorter than that of control crickets injected with Discosoma sp. Red2 (DsRed2) dsRNA. These results suggest that in the cricket, tim plays some role in fine-tuning of the free-running period but may not be essential for oscillation of the circadian clock.Mammalian homologs for timeless
In 1998, researchers identified a mouse homolog and a human homolog of the Drosophila timeless gene. The exact role of TIM in mammals is still unclear, as Tim transcription does not oscillate rhythmically and the TIM protein remains in the nucleus. Moreover, mammalian tim is more orthologous the Tim-2 (Timeout) paralog of the Drosophila Timeless gene. The function of Timeout has yet to be identified.The timeless protein is thought to directly connect the cell cycle with the circadian rhythm in mammals. In this model called a “direct coupling” the two cycles share a key protein whose expression exhibits a circadian pattern.
Mice
Recent work on the mammalian timeless (mTim) in mice has suggested that the genes identified may not play the same essential role in mammals as in DrosophilaDrosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
as an essential function of the circadian clock. mTim is expressed in the suprachiasmatic nucleus
Suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...
(SCN), but there is no oscillation of its accompanying RNA or protein products in constant conditions. The mammalian TIM protein levels do not shift with light signals, but there is reported interaction with the mammalian period protein PER1
PER1
Period circadian protein homolog 1 is a protein that in humans is encoded by the PER1 gene....
and mammalian cryptochrome
Cryptochrome
Cryptochromes are a class of blue light-sensitive flavoproteins found in plants and animals. Cryptochromes are involved in the circadian rhythms of plants and animals, and in the sensing of magnetic fields in a number of species...
(CRY1 and CRY2). mTim is shown to be necessary for embryonic development in mice, indicating a different gene function than in Drosophila This suggests a divergence between mammalian clocks and the Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
clock.
Humans
The human timeless protein (hTIM) has been shown to be required for the production of electrical oscillations output by the suprachiasmatic nucleusSuprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...
(SCN), the major clock governing all tissue-specific circadian rhythms of the body. This protein also interacts with the products of major clock genes CLOCK
CLOCK
Clock is a gene encoding a basic-helix-loop-helix-PAS transcription factor that affects both the persistence and period of circadian rhythms...
, BMAL
BMAL
Bmal is a gene that encodes proteins regulating circadian rhythm. BMAL proteins form part of a basic-helix-loop-helix transcription factor. BMAL-1 dimerizes with CLOCK in vivo and transactivates gene expression of Period and Timeless in drosophila by binding to E-box elements in their promoters...
, PER1
PER1
Period circadian protein homolog 1 is a protein that in humans is encoded by the PER1 gene....
, PER2
PER2
Period circadian protein homolog 2 is a protein that in humans is encoded by the PER2 gene.PER2 contains glucocorticoid response elements and a GRE within the core clock gene Per2 is continuously occupied during rhythmic expression and essential for glucocorticoid regulation of PER2 in vivo...
and PER3
PER3
Period circadian protein homolog 3 is a protein that in humans is encoded by the PER3 gene....
.
Sancar and colleagues investigated whether hTIM played a similar role to orthologs in C. elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...
and other types of yeast, which are known to play play important roles in the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
. Their experiments suggested that hTIM plays an integral role in the G2/M
Cell cycle checkpoint
Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in eukaryotic cells. These checkpoints verify whether the processes at each phase of the cell cycle have been accurately completed before progression into the next phase...
and intra-S cell cycle checkpoints. With respect to the G2/M checkpoint, hTIM binds to the ATRIP subunit on ATR
Ataxia telangiectasia and Rad3 related
Serine/threonine-protein kinase ATR also known as ataxia telangiectasia and Rad3-related protein or FRAP-related protein 1 is an enzyme that in humans is encoded by the ATR gene...
– a protein kinase sensitive to DNA damage. This binding between hTIM and ATR then leads to the phosphorylation of Chk1, resulting in cell cycle arrest or apoptosis. This process serves as an important control to stop the proliferation of cells with DNA damage prior to mitotic
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
division. The role of hTIM in the intra-S checkpoint is less clear at the molecular level. However, down-regulation of hTIM leads to an increase in the rate of generation of replication forks – even in the presence of DNA damage and other regulatory responses.
See also
- Clock geneCLOCKClock is a gene encoding a basic-helix-loop-helix-PAS transcription factor that affects both the persistence and period of circadian rhythms...
- Period genePeriod (gene)Period is a gene located on the X chromosome of Drosophila melanogaster. Oscillations in levels of both per transcript and its corresponding protein PER have a period of approximately 24 hours and together play a central role in the molecular mechanism of the Drosophila biological clock driving...
- Suprachiasmatic nucleusSuprachiasmatic nucleusThe suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...
- Oscillating geneOscillating geneIn molecular biology, an oscillating gene or clock gene is a gene that is expressed in an oscillating pattern, often circadian....