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Publications

2003
Huq E, Al-Sady B, Quail PH. Nuclear translocation of the photoreceptor phytochrome B is necessary for its biological function in seedling photomorphogenesis. Plant J. 35 (5) :660-4.Abstract
The phytochrome (phy) family of sensory photoreceptors (phyA to phyE in Arabidopsis) enables plants to optimize their growth and development under natural light environments. Subcellular localization studies have shown that the photoreceptor molecule is induced to translocate from cytosol to nucleus by light, but direct evidence of the functional relevance of this translocation has been lacking. Here, using a glucocorticoid receptor-based fusion protein system, we demonstrate that both photoactivation and nuclear translocation combined are necessary and sufficient for the biological function of phyB. Conversely, neither artificial nuclear translocation of non-photoactivated phyB nor artificial retention of photoactivated phyB in the cytosol provides detectable biological activity. Together these data indicate that signal transfer from photoactivated phyB to its primary signaling partner(s) is localized in the nucleus, and conversely suggest the absence of a cytosolic pathway from photoactivated phyB to light-responsive genes.
Bailey PC, Martin C, Toledo-Ortiz G, Quail PH, Huq E, Heim MA, Jakoby M, Werber M, Weisshaar B. Update on the basic helix-loop-helix transcription factor gene family in Arabidopsis thaliana. Plant Cell. 15 (11) :2497-502.
2002
Shimizu-Sato S, Huq E, Tepperman JM, Quail PH. A light-switchable gene promoter system. Nat Biotechnol. 20 (10) :1041-4.Abstract
Regulatable transgene systems providing easily controlled, conditional induction or repression of expression are indispensable tools in biomedical and agricultural research and biotechnology. Several such systems have been developed for eukaryotes. Most of these rely on the administration of either exogenous chemicals or heat shock. Despite the general success of many of these systems, the potential for problems, such as toxic, unintended, or pleiotropic effects of the inducing chemical or treatment, can impose limitations on their use. We have developed a promoter system that can be induced, rapidly and reversibly, by short pulses of light. This system is based on the known red light-induced binding of the plant photoreceptor phytochrome to the protein PIF3 and the reversal of this binding by far-red light. We show here that yeast cells expressing two chimeric proteins, a phytochrome-GAL4-DNA-binding-domain fusion and a PIF3-GAL4-activation-domain fusion, are induced by red light to express selectable or "scorable" marker genes containing promoters with a GAL4 DNA-binding site, and that this induction is rapidly abrogated by subsequent far-red light. We further show that the extent of induction can be controlled precisely by titration of the number of photons delivered to the cells by the light pulse. Thus, this system has the potential to provide rapid, noninvasive, switchable control of the expression of a desired gene to a preselected level in any suitable cell by simple exposure to a light signal.
Huq E, Quail PH. PIF4, a phytochrome-interacting bHLH factor, functions as a negative regulator of phytochrome B signaling in Arabidopsis. EMBO J. 21 (10) :2441-50.Abstract
Plants sense and respond to red and far-red light using the phytochrome (phy) family of photoreceptors. However, the mechanism of light signal transduction is not well defined. Here, we report the identification of a new mutant Arabidopsis locus, srl2 (short under red-light 2), which confers selective hypersensitivity to continuous red, but not far-red, light. This hypersensitivity is eliminated in srl2phyB, but not srl2phyA, double mutants, indicating that this locus functions selectively and negatively in phyB signaling. The SRL2 gene encodes a bHLH factor, designated PIF4 (phytochrome-interacting factor 4), which binds selectively to the biologically active Pfr form of phyB, but has little affinity for phyA. Despite its hypersensitive morphological phenotype, the srl2 mutant displays no perturbation of light-induced expression of marker genes for chloroplast development. These data suggest that PIF4 may function specifically in a branch of the phyB signaling network that regulates a subset of genes involved in cell expansion. Consistent with this proposal, PIF4 localizes to the nucleus and can bind to a G-box DNA sequence motif found in various light-regulated promoters.
2000
Martı́nez-Garcı́a JF, Huq E, Quail PH. Direct Targeting of Light Signals to a Promoter Element-Bound Transcription Factor. Science [Internet]. 288 (5467) :859-863. Publisher's VersionAbstract
Light signals perceived by the phytochrome family of sensory photoreceptors are transduced to photoresponsive genes by an unknown mechanism. Here, we show that the basic helix-loop-helix transcription factor PIF3 binds specifically to a G-box DNA-sequence motif present in various light-regulated gene promoters, and that phytochrome B binds reversibly to G-box–bound PIF3 specifically upon light-triggered conversion of the photoreceptor to its biologically active conformer. We suggest that the phytochromes may function as integral light-switchable components of transcriptional regulator complexes, permitting continuous and immediate sensing of changes in this environmental signal directly at target gene promoters.
Huq E, Tepperman JM, Quail PH. GIGANTEA Is a Nuclear Protein Involved in Phytochrome Signaling in Arabidopsis. PNAS [Internet]. 97 (17) :9789 - 9794. Publisher's VersionAbstract

In a genetic screen of available T-DNA-mutagenized Arabidopsis populations for loci potentially involved in phytochrome (phy) signaling, we identified a mutant that displayed reduced seedling deetiolation under continuous red light, but little if any change in responsiveness to continuous far-red light. This behavior suggests disruption of phyB, but not phyA signaling. We have cloned the mutant locus by using the T-DNA insertion and found that the disrupted gene is identical to the recently described GIGANTEA (GI) gene identified as being involved in control of flowering time. The encoded GI polypeptide has no sequence similarity to any known proteins in the database. However, by using β -glucuronidase-GI and green fluorescent protein-GI fusion constructs, we have shown that GI is constitutively targeted to the nucleus in transient transfection assays. Optical sectioning by using the green fluorescent protein-GI fusion protein showed green fluorescence throughout the nucleoplasm. Thus, contrary to previous computer-based predictions that GI would be an integral plasmamembrane-localized polypeptide, the data here indicate that it is a nucleoplasmically localized protein. This result is consistent with the proposed role in phyB signaling, given recent evidence that early phy signaling events are nuclear localized.

Huq E, Kang Y, Halliday KJ, Qin MM, Quail PH. SRL1: a new locus specific to the phyB-signaling pathway in Arabidopsis. Plant Journal. 23 (4) :461-470.Abstract

As part of an effort to isolate new Arabidopsis mutants specifically defective in responsiveness to red light, we identified srl1 (short hypocotyl in red light) by screening an EMS-mutagenized M-2 population derived from a phytochrome B (phyB)-overexpressor line (ABO). The srl1 mutant shows enhanced responsiveness to continuous red but not far-red light, in both wild-type and ABO backgrounds, consistent with involvement in the phyB-signaling pathway but not that of phyA. The hypersensitive phenotype of srl1 is not due to overexpression of endogenous phyA or phyB, and the locus maps to the center of chromosome 2, distinct from any other known photomorphogenic mutants. srl1 seedlings display enhancement of several phyB-mediated responses, including shorter hypocotyls, more expanded cotyledons, shorter petioles and modestly higher levels of CAB gene expression under red light than the wild type. Double mutant analyses show that the hypersensitive phenotype of srl1 is completely phyB-dependent. The data suggest, therefore, that SRL1 may encode a negatively acting component specific to the phyB-signaling pathway.

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