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Steve A Kay

Publications and source records attributed to Steve A Kay.

At least 37 records · Page 2Linked to original sources
PubMed2003

Targeted degradation of TOC1 by ZTL modulates circadian function in Arabidopsis thaliana.

The underlying mechanism of circadian rhythmicity appears to be conserved among organisms, and is based on negative transcriptional feedback loops forming a cellular oscillator (or 'clock'). Circadian changes in protein stability, phosphorylation and subcellular localization also contribute to the generation and maintenance of this clock. In plants, several genes have been shown to be closely associated with the circadian system. However, the molecular mechanisms proposed to regulate the plant clock are mostly based on regulation at the transcriptional level. Here we provide genetic and molecular evidence for a role of ZEITLUPE (ZTL) in the targeted degradation of TIMING OF CAB EXPRESSION 1 (TOC1) in Arabidopsis thaliana (thale cress). The physical interaction of TOC1 with ZTL is abolished by the ztl-1 mutation, resulting in constitutive levels of TOC1 protein expression. The dark-dependent degradation of TOC1 protein requires functional ZTL, and is prevented by inhibiting the proteosome pathway. Our results show that the TOC1-ZTL interaction is important in the control of TOC1 protein stability, and is probably responsible for the regulation of circadian period by the clock.

Arabidopsis↗
PubMed2003

The F box protein AFR is a positive regulator of phytochrome A-mediated light signaling.

Light is an important environmental cue to plants, and much of their physiology is influenced by light. The light signals that drive these responses are perceived by photoreceptors including the red/far-red responsive phytochromes (phyA-E). In addition to direct effects, light also exerts its influence by modifying the rhythms generated by the circadian clock. In Arabidopsis thaliana, the molecular makeup of the interface between the central clock and its input/output pathways is not fully defined, but a major point of control is likely to be protein turnover mediated by the ubiquitin/26S proteasome system. To identify additional constituents of this interface, stable double-stranded RNA interference (RNAi) was used to reduce mRNA levels of rhythmically expressed candidate genes encoding putative components of E3 ubiquitin ligases (i.e., F box and RING finger proteins), followed by screening of the transgenic plants for circadian and light signaling defects. RNAi lines with diminished expression of the novel gene ATTENUATED FAR-RED RESPONSE (AFR) display phenotypes consistent with impaired phyA-mediated light signaling. Furthermore, AFR is a true SCF E3 ubiquitin ligase component. SCF(AFR) is expected to mediate the turnover of a repressor of phyA signaling, possibly to prepare the plant to receive light signals at dawn.

Arabidopsis↗
PubMed2003

A genomic analysis of the shade avoidance response in Arabidopsis.

Plants respond to the proximity of neighboring vegetation by elongating to prevent shading. Red-depleted light reflected from neighboring vegetation triggers a shade avoidance response leading to a dramatic change in plant architecture. These changes in light quality are detected by the phytochrome family of photoreceptors. We analyzed global changes in gene expression over time in wild-type, phyB mutant, and phyA phyB double mutant seedlings of Arabidopsis in response to simulated shade. Using pattern fitting software, we identified 301 genes as shade responsive with patterns of expression corresponding to one of various physiological response modes. A requirement for a consistent pattern of expression across 12 chips in this way allowed more subtle changes in gene expression to be considered meaningful. A number of previously characterized genes involved in light and hormone signaling were identified as shade responsive, as well as several putative, novel shade-specific signal transduction factors. In addition, changes in expression of genes in a range of pathways associated with elongation growth and stress responses were observed. The majority of shade-responsive genes demonstrated antagonistic regulation by phyA and phyB in response to shade following the pattern of many physiological responses. An analysis of promoter elements of genes regulated in this way identified conserved promoter motifs potentially important in shade regulation.

Arabidopsis↗
PubMed2003

FKF1 is essential for photoperiodic-specific light signalling in Arabidopsis.

Adaptation to seasonal change is a crucial component of an organism's survival strategy. To monitor seasonal variation, organisms have developed the capacity to measure day length (photoperiodism). Day-length assessment involves the photoperiodic control of flowering in Arabidopsis thaliana, whereby the coincidence of light and high expression of CONSTANS (CO) induces the expression of FLOWERING LOCUS T (FT), leading to flowering in long-day conditions. Although controlling CO expression is clearly a key step in day-length discrimination, the mechanism that generates day-length-dependent CO expression remains unknown. Here we show that the clock-controlled FLAVIN-BINDING, KELCH REPEAT, F-BOX (FKF1) protein has an essential role in generating the diurnal CO peak and that this function is dependent on light. We show that a recombinant FKF1 LIGHT, OXYGEN OR VOLTAGE (LOV) domain binds the chromophore flavin mononucleotide and undergoes light-induced photochemistry, indicating that FKF1 may function as a photoperiodic blue-light receptor. It is likely that the circadian control of FKF1 expression and the light regulation of FKF1 function coincide to control the daytime CO waveform precisely, which in turn is crucial for day-length discrimination by Arabidopsis.

Arabidopsis↗
PubMed2003

HY5, Circadian Clock-Associated 1, and a cis-element, DET1 dark response element, mediate DET1 regulation of chlorophyll a/b-binding protein 2 expression.

DET1 is a pleiotropic regulator of Arabidopsis development and controls the expression of many light-regulated genes. To gain a better understanding of the mechanism by which DET1 controls transcription from light-regulated promoters, we identified elements in the chlorophyll a/b-binding protein 2 (CAB2) promoter that are required for DET1-mediated expression. Using a series of reporter constructs in which the luciferase gene is controlled by CAB2 promoter fragments, we defined two DET1-responsive elements in the CAB2 promoter that are essential for proper CAB2 transcription. A 40-bp DET1 dark-response element (DtRE) is required for both dark and root-specific repression of CAB2, whereas the known CAB upstream factor-1 element is required for DET1 activation-associated effects in the light and repression in the roots. HY5, a factor that binds CAB upstream factor-1, is also required for DET1 effects in the light. DtRE binds two distinct activities in Arabidopsis seedling extracts: a novel activity with binding site CAAAACGC that we have named CAB2 DET1-associated factor 1 plus an activity that is likely to be the myb transcription factor Circadian Clock-Associated 1. Both activities are altered in dark-grown det1 extracts as compared with wild type, correlating a change in extractable DNA binding activity with a major change in CAB2 expression. We conclude that DET1 represses the CAB2 promoter in the dark by regulating the binding of two factors, CAB2 DET1-associated factor 1 and Circadian Clock-Associated 1, to the DtRE.

Animals↗
PubMed2003

Circadian clocks in daily and seasonal control of development.

The rotation of the earth results in regular changes in the light environment, and organisms have evolved a molecular oscillator that allows them to anticipate these changes. This daily molecular oscillator, known as the circadian clock, regulates a diverse array of physiologies across a wide variety of organisms. This review highlights a few of the insights we have into circadian clock regulation of development, in both plants and animals. A common thread linking plants and animals is the use of the circadian clock to sense changes in day length and to mediate a diverse number of photoperiodic responses.

Animals↗
PubMed2003

Melanopsin is required for non-image-forming photic responses in blind mice.

Although mice lacking rod and cone photoreceptors are blind, they retain many eye-mediated responses to light, possibly through photosensitive retinal ganglion cells. These cells express melanopsin, a photopigment that confers this photosensitivity. Mice lacking melanopsin still retain nonvisual photoreception, suggesting that rods and cones could operate in this capacity. We observed that mice with both outer-retinal degeneration and a deficiency in melanopsin exhibited complete loss of photoentrainment of the circadian oscillator, pupillary light responses, photic suppression of arylalkylamine-N-acetyltransferase transcript, and acute suppression of locomotor activity by light. This indicates the importance of both nonvisual and classical visual photoreceptor systems for nonvisual photic responses in mammals.

Animals↗
PubMed2003

Genome-wide single-nucleotide polymorphism analysis defines haplotype patterns in mouse.

The nature and organization of polymorphisms, or differences, between genomes of individuals are of great interest, because these variations can be associated with or even underlie phenotypic traits, including disease susceptibility. To gain insight into the genetic and evolutionary factors influencing such biological variation, we have examined the arrangement (haplotype) of single-nucleotide polymorphisms across the genomes of eight inbred strains of mice. These analyses define blocks of high or low diversity, often extending across tens of megabases that are delineated by abrupt transitions. These observations provide a striking contrast to the haplotype structure of the human genome.

Animals↗
PubMed2003

Living by the calendar: how plants know when to flower.

Reproductive processes in plants and animals are usually synchronized with favourable seasons of the year. It has been known for 80 years that organisms anticipate seasonal changes by adjusting developmental programmes in response to daylength. Recent studies indicate that plants perceive daylength through the degree of coincidence of light with the expression of CONSTANS, which encodes a clock-regulated transcription factor that controls the expression of floral-inductive genes in a light-dependent manner.

Circadian Rhythm↗
PubMed2003

Dual role of TOC1 in the control of circadian and photomorphogenic responses in Arabidopsis.

To examine the role of the TOC1 (TIMING OF CAB EXPRESSION1) gene in the Arabidopsis circadian system, we generated a series of transgenic plants expressing a gradation in TOC1 levels. Silencing of the TOC1 gene causes arrhythmia in constant darkness and in various intensities of red light, whereas in blue light, the clock runs faster in silenced plants than in wild-type plants. Increments in TOC1 gene dosage delayed the pace of the clock, whereas TOC1 overexpression abolished rhythmicity in all light conditions tested. Our results show that TOC1 RNA interference and toc1-2 mutant plants displayed an important reduction in sensitivity to red and far-red light in the control of hypocotyl elongation, whereas increments in TOC1 gene dosage clearly enhanced light sensitivity. Furthermore, the red light-mediated induction of CCA1/LHY expression was decreased in TOC1 RNA interference and toc1-2 mutant plants, indicating a role for TOC1 in the phytochrome regulation of circadian gene expression. We conclude that TOC1 is an important component of the circadian clock in Arabidopsis with a crucial function in the integration of light signals to control circadian and morphogenic responses.

Arabidopsis↗
PubMed2003

DNA arrays: applications and implications for circadian biology.

DNA arrays can be powerful tools to investigate transcriptional regulation of biological systems in a highly parallel and genome-wide manner. Their use has furthered the investigation of basic cell and developmental biology and is now being applied toward the understanding and diagnosis of human disease. DNA arrays are of particular use in the study of circadian biology because the clock is, at its heart, a transcriptional/translational feedback loop. Here, we review the underlying technology behind DNA arrays, discuss general applications, and focus on the use of the arrays in the study of circadian biology in plants, flies, and mice.

Animals↗
PubMed2003

Circadian light input in plants, flies and mammals.

The rotation of our planet results in daily changes in light and darkness, as well as seasons with characteristic photoperiods. Adaptation to these daily and seasonal changes in light properties (and associated changes in the environment) is important to the sustained survival of higher life forms on our planet. Many organisms use their intrinsic circadian oscillator or clock to orchestrate daily rhythms in behaviour and physiology to adapt to diurnal changes. Some higher organisms use the same oscillator to monitor day length in selecting the appropriate season for reproductive behaviour. Organisms have developed irradiance measurement mechanisms to ignore photic noise (lightning, moonlight), and use the light of dusk and dawn for circadian photoentrainment. They have also devised multiple photoreceptors and signalling cascades to buffer against changes in the spectral composition of natural light. The interaction of the clock with ambient light is, therefore, quite intricate.

Animals↗
PubMed2002

Melanopsin (Opn4) requirement for normal light-induced circadian phase shifting.

The master circadian oscillator in the hypothalamic suprachiasmatic nucleus is entrained to the day/night cycle by retinal photoreceptors. Melanopsin (Opn4), an opsin-based photopigment, is a primary candidate for photoreceptor-mediated entrainment. To investigate the functional role of melanopsin in light resetting of the oscillator, we generated melanopsin-null mice (Opn4-/-). These mice entrain to a light/dark cycle and do not exhibit any overt defect in circadian activity rhythms under constant darkness. However, they display severely attenuated phase resetting in response to brief pulses of monochromatic light, highlighting the critical role of melanopsin in circadian photoentrainment in mammals.

Animals↗
PubMed2002

Genome-wide expression analysis in Drosophila reveals genes controlling circadian behavior.

In Drosophila, a number of key processes such as emergence from the pupal case, locomotor activity, feeding, olfaction, and aspects of mating behavior are under circadian regulation. Although we have a basic understanding of how the molecular oscillations take place, a clear link between gene regulation and downstream biological processes is still missing. To identify clock-controlled output genes, we have used an oligonucleotide-based high-density array that interrogates gene expression changes on a whole genome level. We found genes regulating various physiological processes to be under circadian transcriptional regulation, ranging from protein stability and degradation, signal transduction, heme metabolism, detoxification, and immunity. By comparing rhythmically expressed genes in the fly head and body, we found that the clock has adapted its output functions to the needs of each particular tissue, implying that tissue-specific regulation is superimposed on clock control of gene expression. Finally, taking full advantage of the fly as a model system, we have identified and characterized a cycling potassium channel protein as a key step in linking the transcriptional feedback loop to rhythmic locomotor behavior.

Animals↗
PubMed2002

Molecular basis of seasonal time measurement in Arabidopsis.

Several organisms have evolved the ability to measure daylength, or photoperiod, allowing them to adjust their development in anticipation of annual seasonal changes. Daylength measurement requires the integration of temporal information, provided by the circadian system, with light/dark discrimination, initiated by specific photoreceptors. Here we demonstrate that in Arabidopsis this integration takes place at the level of CONSTANS (CO) function. CO is a transcriptional activator that accelerates flowering time in long days, at least in part by inducing the expression of FLOWERING LOCUS T (FT). First, we show that precise clock control of the timing of CO expression, such that it is high during daytime only in long days, is critical for daylength discrimination. We then provide evidence that CO activation of FT expression requires the presence of light perceived through cryptochrome 2 (cry2) or phytochrome A (phyA). We conclude that an external coincidence mechanism, based on the endogenous circadian control of CO messenger RNA levels, and the modulation of CO function by light, constitutes the molecular basis for the regulation of flowering time by daylength in Arabidopsis.

Arabidopsis↗
PubMed2002

Circadian rhythms from flies to human.

In this era of jet travel, our body 'remembers' the previous time zone, such that when we travel, our sleep wake pattern, mental alertness, eating habits and many other physiological processes temporarily suffer the consequences of time displacement until we adjust to the new time zone. Although the existence of a circadian clock in humans had been postulated for decades, an understanding of the molecular mechanisms has required the full complement of research tools. To gain the initial insights into circadian mechanisms, researchers turned to genetically tractable model organisms such as Drosophila.

Animals↗
PubMed2002

Coordinated transcription of key pathways in the mouse by the circadian clock.

In mammals, circadian control of physiology and behavior is driven by a master pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. We have used gene expression profiling to identify cycling transcripts in the SCN and in the liver. Our analysis revealed approximately 650 cycling transcripts and showed that the majority of these were specific to either the SCN or the liver. Genetic and genomic analysis suggests that a relatively small number of output genes are directly regulated by core oscillator components. Major processes regulated by the SCN and liver were found to be under circadian regulation. Importantly, rate-limiting steps in these various pathways were key sites of circadian control, highlighting the fundamental role that circadian clocks play in cellular and organismal physiology.

Animals↗