16The ZEITLUPE Family of Putative Photoreceptors

Thomas F. Schultz

16.1Introduction

Genetic analysis of the circadian system in Arabidosis has led to the identification ofa novel family of photoreceptors, the ZTL family. Molecular genetic analysis of thisfamily of proteins indicates that its members play key roles within the circadian sys-tem. Members of this family appear to be unique in possessing protein motifs thatsuggest they perceive light and feed light signals into the circadian clock via light-de-pendent changes in protein ubiquitination. The goal is this chapter is to summarizethe data on the ZTL family members and to integrate these data into a model for theirmode of action within the circadian system of Arabidopsis.

16.2Circadian Clocks

Due to the rotation of the earth, organisms have evolved under continuous f luctua-tions in light and temperature environments. These f luctuations occur with a 24-hour rhythmicity and organisms spanning all major kingdoms have evolved a circa-dian clock allowing them to anticipate these regular and constant changes in their en-vironment. The hallmark of processes under circadian control is that they continueto exhibit rhythmic behavior with an approximate 24-hour periodicity even whentransferred to constant conditions. The rhythmic control of these processes has fas-cinated biologists for more than 100 years since their first descriptions. The circadi-an system consists of three broad domains: input or entrainment pathways, the cen-tral oscillator which generates the overt rhythms, and processes that are under circa-dian control, also known as outputs. Recent advances in the field of circadian biolo-gy has led to molecular insights into circadian mechanisms including theidentification of the ZTL gene family.

In order to understand the functions of the ZTL family fully, a brief description ofthe circadian clock is necessary. Current models of circadian oscillators in all organ-

Handbook of Photosensory Receptors. Edited by W. R. Briggs, J. L. SpudichCopyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN 3-527-31019-3

338

isms consist of feedback loops based on transcription and translation (Young et al.,2001). Positive factors drive expression of a repressor at a constant rate. The repres-sor accumulates within the cell until it reaches a threshold level. Once this thresholdis reached, the repressor feeds back and suppresses its own expression. Once its pro-tein levels fall below the threshold, the positive factors enhance its accumulation andstart the cycle again. This simplified feedback model affords multiple control pointsthat are able to affect the pace at which the clock progresses through its cycle. InDrosophila, PER and TIM form the negative arm of the feedback loop and functionto repress their own promoters. Their activities are regulated by entry into the nucle-us, phosphorylation, and protein stability (Panda et al., 2002), and alterations in anyof these control points causes changes in the pace at which the clock progresses. InArabidopsis, a feedback loop has been described between two Myb-domain transcrip-tion factors (the repressors CCA1 and LHY) and the TOC1 protein (Figure 16.1), apseudo-response regulator protein (Alabadi et al., 2001). Phosphorylation of CCA1has been shown to regulate the pace at which the clock runs and alterations in pro-tein stability has been described for both LHY and TOC1 (Kim et al., 2003; Mas et al.,2003a; Sugano et al., 1999). Multiple control points exist in all circadian systems ana-lyzed and these control points function to maintain a 24-hour periodicity and are uti-lized to make the oscillator entrainable to environmental stimuli such as light andtemperature.

Entrainment pathways function to sense environmental signals such as changes inlight and temperature and feed these signals into the circadian oscillator. These path-ways are not considered part of the oscillator itself but are crucial for the oscillator tomaintain a stable relationship between itself and its environment. The mechanismsby which temperature entrainment occurs are largely unknown, while light entrain-ment has been at least partially elucidated (Devlin and Kay, 2001). In mammalian sys-tems, clock entrainment appears to occur via ocular light perception. Removal of the

16 The ZEITLUPE Family of Putative Photoreceptors

TOC1

OscillatorInput Outputs

Light

Temperature

ProteosomeZTLLKP2

TOC1

FKF1CO

FT

Flowering

CCA1LHY

Phy’sCry’s

Figure 16.1 Schematic representation showing the positions occu-pied by ZTL family members within the circadian system.

339

eyes results in a loss of the ability to entrain the circadian clock (Foster et al., 1991).However, classical ocular photoreceptors, the rhodopsins, do not appear to be the on-ly photoreceptors mediating entrainment. Mutations that abolish light perception viarhodopsins resulting in complete blindness (rd mutants) do not abolish the ability toentrain the circadian clock (Foster et al., 1991). Recently a novel gene related to opsinswas identified and named melanopsin (Provencio et al., 2000). This photoreceptor ex-hibits proper cell-specific expression profiles and appears to function in light inputto the clock. However, loss of melanopsin alone does not appear to abolish circadianphotoperception either, indicating that multiple photoreceptors constitute this func-tion (Panda et al., 2003). Melanopsin in combination with the rhodopsins appear toform the basis for circadian photoperception in mammalian systems, although morecircadian photoreceptors may continue to be discovered. The mammalian cryp-tochromes have also been described as candidate circadian photoreceptors and theymay play a role in phototransduction to the mammalian circadian clock, although adirect role in light perception has yet to be established (see Van Gelder and Sancar,Chapter 12).

Unlike mammalian systems, plants do not exhibit tissue-specific light perceptionand all plants cells appear to possess the ability to detect light. However, as with mam-malian systems, plants also utilize multiple photoreceptors for light input to theclock. Known plant photoreceptors, the phytochromes and cryptochromes, appear toplay key roles in circadian photoperception (Devlin and Kay, 2000; Somers et al.,1998). Phytochromes are the classical red-light photoreceptors in plants and consistof multigene families. There are five phytochromes present in the Arabidopsisgenome. The two most prominent and well-studied are phyA and phyB. Loss of ei-ther of these phytochromes results in period lengthening under constant red-light.phyB appears to be the predominant red-light photoreceptor at higher f luences andphyA appears to be predominant at lower f luences. Having multiple red-light pho-toreceptors allows the plant to respond to a much wider range of f luences, a strategythat is also utilized for blue-light photoperception.

In Arabidopsis, the cry1 and cry2 genes constitute the cryptochrome gene familyand function in blue-light perception. Again the presence of two blue-light photore-ceptors appears to allow the plant to respond to a much wider range of f luences. Thesimilarities between the plant and mammalian entrainment systems are worth not-ing. Neither system uses a single circadian photoreceptor and both systems employmultiple receptors allowing responses to various light qualities and quantities. Theevolution of such an elaborate and redundant system of perception suggests a crucialrole for clock entrainment in the fitness of the organism. As further analysis of clockentrainment proceeds, novel mechanisms of photoperception will certainly continueto be discovered.

16.2 Circadian Clocks

340

16.3SCF Ubiquitin Ligases

Ubiquitination has emerged as key mechanism for regulating numerous cellularprocesses such as transcription, signal transduction, cell division, and development.Ubiquitin molecules are transferred onto target proteins via a cascade of steps fromE1 to E2 to E3 ubiquitin ligases (Hershko and Ciechanover, 1998). The E3 ubiquitinligases confer specificity onto the system via protein: protein interactions with targetmolecules. E3 ubiquitin ligases represent an incredibly diverse family of proteins andhave been categorized into five general classes (Vierstra, 2003): HECT-domain pro-teins, SCF protein complexes, RING/U-box, APC, and VBC-Cul2 proteins. Analysisof the Arabidopsis genome has demonstrated the importance of SCF (Skp, Cullin, F-box) complexes in plant biology. The yeast Cyclin F protein was the founding mem-ber of the F-box motif. The function of F-box proteins is to confer specificity onto SCFcomplexes by interacting with target proteins. Nearly 700 putative F-box proteins areencoded in the Arabidopsis genome (Gagne et al., 2002). This number representsnearly 2.5% of the genome and contains twice as many as the RING-finger class ofE3 ubiquitin ligases, the second largest group in Arabidopsis. The staggering numberof these proteins indicates the importance plants have placed on this family of ubiq-uitin ligases.

The ZTL family of proteins belongs to the SCF class of E3 ubiquitin ligases. Mem-bers of the ZTL family contain an F-box domain and six kelch repeats at their C-ter-mini (Figure 16.2) (Nelson et al., 2000; Schultz et al., 2001; Somers et al., 2000). Thekelch domain forms a β-propeller structure, a structurally conserved motif found ina large number of proteins (Adams et al., 2000). Each kelch repeat forms a singleblade of the propeller-like structure and the motif is thought to mediate protein:pro-tein interactions. Out of the nearly 700 Arabidopsis F-box proteins, over 90 have kelchrepeats at their C-termini indicating that F-box/kelch proteins are fairly common.What makes the ZTL family members unique is the presence of a PAS/LOV domainat their N-termini (Figure 16.2), with only three proteins in the Arabidopsis genomeexhibiting this unique architecture. In addition, phylogenetic analysis of all knownArabidopsis F-box proteins groups this family together in a single clade with no ad-ditional members (Gagne et al., 2002). PAS/LOV domains are remarkably diverseand are found in organisms spanning prokaryotes and eukaryotes. Their functionscan be broadly categorized into two groups, they either mediate small molecule bind-ing or participate in protein:protein interactions (Gu et al., 2000) or possibly both.The PAS/LOV domains most closely related to the ZTL family of proteins belongs tothe phototropin1/2 (phto1/2) and WHITE COLLAR-1 (WC-1) proteins (Figure 16.2),both of which are known photoreceptors (see Christie and Briggs, Chapter 13, Swartzand Bogomolni, Chapter 14). The biochemical and physiological analysis of the ZTLfamily members supports their role in photoperception within the circadian system.

16 The ZEITLUPE Family of Putative Photoreceptors

341

16.4Photoperception

As described above, the most striking and unique feature of the ZTL family of pro-teins is the presence of a PAS/LOV domain at the N terminus of an F-box protein.This unusual combination suggests a role for these proteins in light-dependent pro-tein ubiquitination and the physiological and biochemical analysis of this gene fam-ily support this conclusion. The PAS/LOV domains most closely related to the ZTLfamily of proteins belong to the phot1/2 and WC-1/2 proteins, two photoreceptorgene families involved in Arabidopsis phototropism (Christie et al., 1998) and the Neu-rospora circadian clock (Froehlich et al., 2002). Extensive biochemical and photo-chemical analysis of phot1/2 LOV domains has shown that these domains bind f lavinmononucleotide (FMN), a blue light-absorbing chromophore, in a non-covalentmanner. Upon absorption of light, a covalent bond is formed between FMN and aconserved Cys residue within the LOV domain (see Christie and Briggs, Chapter 13and Crosson, Chapter 15 for further details). Furthermore, this covalent bond is la-bile and spontaneously reverts to a non-activated state very rapidly in the photore-ceptors mentioned above. These results indicate that the LOV domains of phot1/2

16.4 Photoperception

1

1

1

609

611

619

ZTL

FKF1

LKP2

PAS/LOV

PAS/LOV

PAS/LOV

F-box

F-box

F-box

kelch

kelch

kelch

*** * * ** * * *

412 483420 430 440 450 460 470(412)FEMVTGYRAEEVLGGNCRFLQCRGPFAKR--RHPLVDSMVVSEIRKCIDEGIEFQGELLNFRKDGSPLMNRLZTL (66)FEVFTGYRADEVLGRNCRFLQYRDPRAQR--RHPLVDPVVVSEIRRCLEEGIEFQGELLNFRKDGTPLVNRLFKF1 (75)FEIVTGYRAEEVIGRNCRFLQCRGPFTKR--RHPMVDSTIVAKMRQCLENGIEFQGELLNFRKDGSPLMNKLLKP 2 (66)FFNMTGYTSKEVVGRNCRFLQGSG-----------TDADELAKIRETLAAGNNYCGRILNYKKDGTSFWNLLp ho t1 (218)FFTMTGYSSKEIVGRNCRFLQGPD-----------TDKNEVAKIRDCVKNGKSYCGRLLNYKKDGTPFWNLLp ho t2 (154)FQNLTGYSRHEIVGRNCRFLQAPDGNVEAGTKREFVENNAVYTLKKTIAEGQEIQQSLINYRKGGKPFLNLLWC-1 (412)

Figure 16.2 Schematic representation of theZTL family members. Numbers underneatheach protein indicate length in amino acids.PAS/LOV sequences are shown for the Ara-bidopsis ZTL family members, phot1/2 pro-

teins, and the Neurospora WC-1 protein. Aster-isks under the sequence denote positions ofFMN-interacting residues as determined bystructural analysis of the Adiantum phy3 pro-tein (Crosson and Moffat, 2001).

342

perform a photocycle and are able to switch into an active state rapidly in response tolight and return to the inactive state in the dark.

Recently, it’s been shown that the LOV domains of all three ZTL family membersexhibit similar although not identical photochemical properties (Imaizumi et al.,2003). FMN co-purifies with these fusion proteins when expressed in bacteria, andthese fusion proteins exhibit photochemical activity. In response to f lashes of bluelight, the proteins exhibit a characteristic shift in absorption spectra consistent withthe formation of a covalent cysteinyl bond similar to that observed for the LOV do-mains of phot1/2. Interestingly, while the LOV domains from phot1/2 spontaneous-ly revert to a resting state quite rapidly in the dark, the covalent bond formed by theZTL family LOV domains appears to be stable, lasting for hours. This result wouldsuggest that either the ZTL family of proteins do not photocycle and are competentto absorb light only once, or in contrast to phot1/2, require an accessory protein(s) fortheir reversion to a non-photoactivated state. Another possibility is that the lack of re-version is a property of bacterially-expressed fusion protein that is not exhibited byfull-length endogenous protein in planta.

16.5The ZTL Gene Family

Members of this gene family (ZTL/FKF1/LKP2, Figure 16.2) were identified usingtwo basic approaches, forward genetics and bioinformatics. The founding memberwas obtained from a forward genetic screen for circadian clock mutants. Mutationsin the ZEITLUPE (ZTL) gene result in a reduction in the pace at which the clock runs(“zeitlupe” is German and roughly translates to “slow motion”) (Somers et al., 2000).The second member, identified concurrently with ZTL, was isolated as a late f lower-ing mutant and was named FKF1 (Flavin-binding, Kelch repeat, F-box) (Nelson et al.,2000). Finally, the LKP2 (LOV, Kelch, Protein2) gene was identified in a search of theArabidopsis genome for putative novel photoreceptors containing a PAS/LOV domain(Schultz et al., 2001). The combination of a PAS/LOV domain with an F-box makesthe members of this gene family unique and stimulated strong interest in theirpotential roles as novel photoreceptors. Consequently, multiple groups identifiedthese proteins independently leading to a number of names being published foreach member. The ZTL gene was identified multiple times and descriptions exist un-der LKP1 (Kiyosue and Wada, 2000), FKF1-like protein 2 (Nelson et al., 2000), andADAGIO1 (ADO1) (Jarillo et al., 2001). FKF1 is also annotated as ADO3 and LKP2 isannotated as ADO2 (Jarillo et al., 2001). For the purposes of this review, each gene willbe referred to as the name with which it first appeared in publication. It should benoted that naming all three based on presumed circadian function could be mislead-ing since FKF1 does not appear to alter the pace at which the clock runs but rathermore specifically regulates f lowering time in response to day length.

16 The ZEITLUPE Family of Putative Photoreceptors

343

16.5.1ZTL

The ZTL gene was first identified in a screen for circadian clock mutants in Ara-bidopsis using luciferase imaging of whole seedlings (Millar et al., 1995). The ztl mu-tants isolated from this type of screen (ztl-1 and ztl-2) both exhibited long-period cir-cadian phenotypes (Somers et al., 2000). Under free-running conditions (constantlight and temperature), CAB2 expression oscillates with an approximate 27-hour pe-riodicity in ztl mutants compared to 24 hours for wild-type plants. The long-periodphenotype was observed for multiple clock outputs (CCR2 expression and leaf move-ment rhythms) suggesting a pervasive effect of the mutation on the circadian clock.Although both mutants exhibited semi-dominant phenotypes in segregating popula-tions, the mutants appear to be loss-of-function and an insertion line with no de-tectable mRNA was extensively characterized and exhibited identical circadian phe-notypes (Jarillo et al., 2001). In contrast to the loss-of-function phenotypes, over-ex-pression of ZTL (gain-of-function) resulted in either arrhythmic or short period phe-notypes (Somers et al., 2004). The severity of the clock phenotype showed a strongcorrelation with ZTL expression levels. Clock phenotypes ranged from short periodto arrhythmicity as ZTL expression levels increased. In addition to circadian pheno-types, both ztl mutants and ZTL over-expressors also exhibited short- and long-hypocotyl phenotypes (respectively), and a slight early f lowering phenotype undershort day growth conditions (ztl mutants) or late f lowering phenotype under longdays (ZTL over-expressors). Both hypocotyl-length and f lowering-time phenotypesare typically observed in clock mutants.

One of the key observations for the ztl-1 and ztl-2 point mutations came from theanalysis of f luence-rate response curves for period length (Somers et al., 2000). Thelong period clock phenotype was dependent on the f luence rate of light. At lower f lu-ences the differential between mutant and wild type period lengths were greater thanat higher f luences. This observation argued strongly for a light-dependent role ofZTL in the circadian clock. Recently it was shown that loss-of-function ztl mutantsexhibit a long period phenotype under constant darkness, a result indicating a morecentral role in the circadian system beyond light input.

The ZTL protein possesses an F-box domain, suggesting that it functions as an E3ubiquitin ligase and targets other proteins for ubiquitination. In searching for candi-date targets of ZTL, an inverse relationship was observed between ztl mutants and an-other key clock gene, TOC1 (Mas et al., 2003b). Reduction in the levels of TOC1 re-sult in short period phenotypes under constant light (Strayer et al., 2000) and in-creases in TOC1 result long period phenotypes (Mas et al., 2003b); these phenotypesmirror the ztl mutant phenotypes leading to the hypothesis that ZTL may targetTOC1 for degradation.

This hypothesis was recently confirmed by demonstrating a direct physical inter-action between these two proteins both in yeast-interaction screens and in plants us-ing co-immunoprecipitation assays (Mas et al., 2003a). Furthermore, TOC1 levels aregreatly increased in ztl mutants and the in vitro degradation of TOC1 was much slow-er in extracts isolated from ztl mutants compared to wild-type extracts. These results

16.5 The ZTL Gene Family

344

support the conclusion that ZTL mediates the ubiquitination and degradation ofTOC1 protein. Furthermore, TOC1 protein is much more stable in the light than inthe dark, suggesting that light inhibits ZTL activity. These results support the modelthat ZTL is a circadian photoreceptor that targets a core clock component (TOC1) fordegradation in the dark (Figure 16.1). This model is consistent with the f luence rate-dependent period-length phenotypes observed for ztl mutants (Somers et al., 2000).At higher f luences ZTL activity in wild-type seedlings is inhibited and TOC1 is morestable such that the differential between mutant and wild-type period lengths issmaller. At lower f luences, wild-type ZTL activity is higher and TOC1 is degradedmore rapidly. The period-length difference between wild type and ztl mutants isgreatest under conditions where ZTL activity is the highest and this appears to be atlower f luence rates. These results indicate that ZTL functions as a clock parameterwithin the circadian system. However it does not participate directly in the oscillatoritself but rather alters the stability of a clock component (TOC1) resulting in alter-ations in the rate at which the clock progresses through its cycle.

16.5.2FKF1

Arabidopsis is a facultative long-day plant f lowering more rapidly under long-daygrowth conditions (16 h light:8 h dark) than under short-day conditions (8 h light:16 hdark). Day-length dependent processes are also known as photoperiodic responses.Classic experiments showed that plants, as well as other organisms, utilize their cir-cadian clocks to sense changes in day-length and the pathway between the circadianclock and the induction of f lowering has been amenable to genetic analysis(Yanovsky and Kay, 2003). Two key proteins have been identified which are central tophotoperiodic control of f lowering, CONSTANS (CO) and FLOWERING LOCUS-T(FT). Expression of FT is a key regulator of the developmental transition to f loweringand is the last committed step before the transition occurs (Kardailsky et al., 1999).CO is a putative transcription factor that directly regulates FT, and the combinationof light at the end of the day and CO expression is required for the induction of FT(Suarez-Lopez et al., 2001; Yanovsky and Kay, 2002). The circadian expression patternof CO is complex and is crucial for its regulation of f lowering time. Under long days,CO expression exhibits a bi-phasic pattern with a peak occurring at approximately10 hours after lights on and a second peak occurring approximately 4 hours afterlights off (Suarez-Lopez et al., 2001). The first peak of expression appears to be cru-cial for the photoperiodic induction of f lowering. When this peak occurs during thelight period (i.e. in long days), CO is able to induce expression of FT which in turn in-duces the transition to f lowering. When this peak occurs after lights off (i.e. in shortdays), CO is unable to induce expression of FT and subsequently f lowering.

The fkf1 mutant was isolated as a large deletion on the bottom of chromosome 1that resulted in a late-f lowering phenotype under long day conditions (Nelson et al.,2000). This late-f lowering phenotype was limited to long days and was not observedin short days indicating a loss of photoperiodic control over time to f lowering. Lossof photoperiodic control of f lowering is common to many clock mutants. In contrast

16 The ZEITLUPE Family of Putative Photoreceptors

345

to ZTL, fkf1 mutants do not appear to have strong circadian phenotypes but do havea much stronger f lowering-time phenotype (Nelson et al., 2000). The expression ofFKF1 is regulated by the circadian clock with peak expression occurring late in theday and its expression may be a direct target of the core clock (Imaizumi et al., 2003).The observations that fkf1 mutants result in a loss of photoperiodic control over f low-ering but do not appear to affect the circadian clock itself, and that FKF1 is clock-reg-ulated, place FKF1 on the output side of the clock on a pathway mediating photope-riodic control over time to f lowering.

Recent results indicate that FKF1 appears to be required for the first peak in CO ex-pression under long days (Imaizumi et al., 2003). In fkf1 mutants, this peak is absentand CO expression is confined to the night. This key result explains the f loweringtime phenotype of fkf1 mutants at the molecular level and place FKF1 on the regula-tory pathway between the circadian clock and photoperiodic control over f lowering.Furthermore, plants entrained in short days respond to extended light during thefirst cycle after transfer to long days and this response requires FKF1. This result sug-gests that FKF1 is required for sensing light and shifting the pattern of CO expres-sion to a long day mode. Thus the phase of FKF1 expression in the presence of lightappears to be crucial for perception of day length and its subsequent regulation oftime to f lowering. FKF1 may be the photoreceptor that mediates the plant’s pho-toperiodic response. However, further biochemical analysis of its mode of action isstill needed.

16.5.3LKP2

The third member of this gene family appears to function within the circadian sys-tem in much the same way as ZTL, although less data are available. Over-expressionof LKP2 results in aberrant clock-regulated expression of CAB2, CCR2, CCA1, LHY,and TOC1 under constant light (Schultz et al., 2001). The pervasive effects on multi-ple clock outputs suggest a central role for LKP2 in the circadian system under con-stant light. The expression of CCR2 in the LKP2 over-expresser was assayed underconstant darkness and also found to be arrhythmic, indicating a light-independentrole for LKP2 in the clock. Over-expression also resulted in long-hypocotyl pheno-types and late f lowering in short days, phenotypes similar to those observed for ZTL.These phenotypes are all limited to gain-of-function (over-expression) studies andloss-of-function alleles have not been described either from forward genetic screensor from insertion lines. These results suggest that ZTL may be the primary circadianphotoreceptor, and while over-expression of LKP2 is able to affect the clock, it mayhave a distinct and undiscovered role within the cell. Further analysis of LKP2, par-ticularly of loss-of-function alleles, is needed to ascertain its function within the Ara-bidopsis circadian system.

16.5 The ZTL Gene Family

346

16.6Summary

The ZTL photoreceptors belong to a small gene family consisting of three members.These genes are unique and possess a photoactive PAS/LOV domain at the N-termi-nus of an F-box protein. Out of nearly 700 F-box proteins in Arabidopsis, only the ZTLfamily members contain a PAS/LOV motif. Genetic and physiological analysis ofZTL indicates that it plays a central role in the circadian system, and molecular analy-sis suggests that it targets a core clock protein (TOC1) for degradation in a light-de-pendent manner (Figure 16.1). The LKP2 protein appears to play a similar role butfurther analysis of this family member is needed to determine its mode of action. TheFKF1 protein appears to function on the output pathway leading from the circadianclock to photoperiodic control of f lowering time. FKF1 is required for proper timingof CO expression, a key f lowering gene, under long days although its mechanism ofaction remains unknown. The PAS/LOV domains of each of these proteins bindFMN and exhibits photochemistry consistent with their roles as photoreceptors thatmediate light-dependent ubiquitination.

References

16 The ZEITLUPE Family of Putative Photoreceptors

Adams, J., Kelso, R., and Cooley, L. (2000)Trends Cell Biol. 10:17–24.

Alabadi, D., Oyama, T., Yanovsky, M. J., Har-mon, F. G., Mas, P., and Kay, S. A. (2001)Science 293:880–883.

Christie, J. M., Reymond, P., Powell, G. K.,Bernasconi, P., Raibekas, A. A., Liscum, E.,and Briggs, W. R. (1998) Science 282:1698–1701.

Crosson, S. and Moffat, K. (2001) Proc. Natl.Acad. Sci. USA 98:2995–3000.

Devlin, P. F. and Kay, S. A. (2000) Plant Cell12:2499–2510.

Devlin, P. F. and Kay, S. A. (2001) Annu. Rev.Physiol. 63:677–694.

Foster, R. G., Provencio, I., Hudson, D.,Fiske, S., De Grip, W., and Menaker, M.(1991) J.Comp. Physiol. [A] 169:39–50.

Froehlich, A. C., Liu, Y., Loros, J. J., and Dun-lap, J. C. (2002) Science 297:815–819.

Gagne, J. M., Downes, B. P., Shiu, S. H., Durs-ki, A. M., and Vierstra, R. D. (2002) Proc.Natl. Acad. Sci. USA 99:11519–11524.

Gu, Y. Z., Hogenesch, J. B., and Bradfield,C. A. (2000) Annu. Rev. Pharmacol. Toxicol.40:519–561.

Hershko, A. and Ciechanover, A. (1998) Annu.Rev. Biochem. 67:425–479.

Imaizumi, T., Tran, H. G., Swartz, T. E.,Briggs, W. R., and Kay, S. A. (2003) Nature426:302–306.

Jarillo, J. A., Capel, J., Tang, R. H., Yang, H. Q.,Alonso, J. M., Ecker, J. R., and Cashmore,A. R. (2001) Nature 410:487–490.

Kardailsky, I., Shukla, V. K., Ahn, J. H., Dage-nais, N., Christensen, S. K., Nguyen, J. T.,Chory, J., Harrison, M. J., and Weigel, D.(1999) Science 286:1962–1965.

Kim, J. Y., Song, H. R., Taylor, B. L., and Carre,I. A. (2003) EMBO J. 22:935–944.

Kiyosue, T. and Wada, M. (2000) Plant J.23:807–815.

Mas, P., Kim, W. Y., Somers, D. E., and Kay,S. A. (2003a) Nature 426:567–570.

Mas, P., Alabadi, D., Yanovsky, M. J., Oyama,T., and Kay, S. A. (2003b) Plant Cell 15:223–236.

Millar, A. J., Carre, I. A., Strayer, C. A., Chua,N. H., and Kay, S. A. (1995) Science267:1161–1163.

Nelson, D. C., Lasswell, J., Rogg, L. E., Cohen,M. A., and Bartel, B. (2000) Cell 101:331–340.

Panda, S., Hogenesch, J. B., and Kay, S. A.(2002) Nature 417:329–335.

347

Panda, S., Provencio, I., Tu, D. C., Pires, S. S.,Rollag, M. D., Castrucci, A. M., Pletcher,M. T., Sato, T. K., Wiltshire, T., Andahazy,M., Kay, S. A., Van Gelder, R. N., and Ho-genesch, J. B. (2003) Science 301:525–527.

Provencio, I., Rodriguez, I. R., Jiang, G.,Hayes, W. P., Moreira, E. F., and Rollag,M. D. (2000) J. Neurosci. 20:600–605.

Schultz, T. F., Kiyosue, T., Yanovsky, M., Wada,M., and Kay, S. A. (2001) Plant Cell 13:2659–2670.

Somers, D. E., Devlin, P. F., and Kay, S. A.(1998) Science 282:1488–1490.

Somers, D. E., Kim, W. Y., and Geng, R. (2004)Plant Cell 16:769–782.

Somers, D. E., Schultz, T. F., Milnamow, M.,and Kay, S. A. (2000) Cell 101:319–329.

Strayer, C., Oyama, T., Schultz, T. F., Raman,R., Somers, D. E., Mas, P., Panda, S., Kreps,

J. A., and Kay, S. A. (2000) Science 289:768–771.

Suarez-Lopez, P., Wheatley, K., Robson, F.,Onouchi, H., Valverde, F., and Coupland, G.(2001) Nature 410:1116–1120.

Sugano, S., Andronis, C., Ong, M. S., Green,R. M., and Tobin, E. M. (1999) Proc. Natl.Acad. Sci. USA 96:12362–12366.

Vierstra, R. D. (2003) Trends Plant Sci. 8:135–142.

Yanovsky, M. J. and Kay, S. A. (2002) Nature419:308–312.

Yanovsky, M. J. and Kay, S. A. (2003) Nat. Rev.Mol. Cell Biol. 4:265–276.

Young, M. W. and Kay, S. A. (2001) Nat. Rev.Genet. 2:702–715.

References