LUX or Phytoclock1 (PCL1) is a gene that codes for LUX ARRHYTHMO, a protein necessary for circadian rhythms in Arabidopsis thaliana. LUX protein associates with Early Flowering 3 (ELF3) and Early Flowering 4 (ELF4) to form the Evening Complex (EC), a core component of the Arabidopsis repressilator model of the plant circadian clock. The LUX protein functions as a transcription factor that negatively regulates PRR9, a core gene of the Midday Complex, another component of the Arabidopsis repressilator model. LUX is also associated with circadian control of hypocotyl growth factors PIF4 and PIF5.
In 2000, the LUX gene was first sequenced in Arabidopsis thaliana by a team at the Plant Gene Expression Center at UC Berkley as a part of the Arabidopsis Genome Initiative. In 2003, scientists from the Plant Gene Expression Center and the Genomic Analysis Laboratory at the Salk Institute for Biological Studies collaborated to identify expression of the LUX gene in Arabidopsis using cDNA arrays. In 2005, scientists at the Center for Gene Research at Nagoya University studied null mutations of LUX and the other Evening Complex genes to show that LUX was necessary for circadian rhythms in A. thaliana.
The LUX gene is located on the third chromosome of Arabidopsis thaliana and contains three exons. Upstream of the LUX gene is a promoter containing a cis-regulatory element known as the "evening element" (EE) with the sequence AAAATATCT. It is overrepresented in evening-expressed genes in the Arabidopsis repressilator. The EE may be bound by CCA1 and LHY to suppress expression of LUX. The LUX ARRHYTHMO protein has a length of 323 amino acids and contains a Myb-like GARP family transcription factor DNA-binding domain.
The LUX ARRHYTHMO gene participates in the regulation of the Arabidopsis thaliana circadian clock. Along with Early Flowering 3 (ELF3) and Early Flowering 4 (ELF4), it is a member of the Evening Complex (EC), a component of the Arabidopsis repressilator model of gene regulation. This three-protein complex is expressed and assembled during the evening to repress transcription of the gene Pseudo-Response Regulator 9 (PRR9), a component of the Midday Complex. LUX likely represses PRR9 via direct binding to a DNA sequence that has not yet been elucidated. PRR9 subsequently represses Circadian Clock Associated 1 (CCA1) and Late Elongated Hypocotyl (LHY), which are other components of the Midday Complex. Although LUX and ELF4 are induced by low intensity, non-damaging UV-B radiation, the direct molecular mechanism of light input into the Arabidopsis circadian clock has yet to be elucidated.
Arabidopsis thaliana Growth and Flowering
The EC binds to promoters of Phytochrome Interacting Factor 4 (PIF4) and Phytochrome Interacting Factor 5 (PIF5), repressing their expression and subsequently inhibiting plant growth in the evening. PIF4 and PIF5 are both basic helix-loop-helix (bHLH) domain transcription factors that are implicated in the induction of Flowering Locus T (FT), a florigen involved in promoting A. thaliana flowering. Mutants lacking functional LUX are unable to repress PIF4 and PIF5, leading to early accumulation of these transcription factors and thus premature growth; consequently, LUX mutants often express an elongated hypocotyl phenotype due to excess growth during the night.
Temperature perception is also related to the Evening Complex. Examination of LUX (as well as ELF3 and ELF4) mutants showed that, despite variations in temperature, the expression of GI (GIGANTEA), LUX, PIF4, PRR7, and PRR9 was constitutively high, suggesting that LUX mutants abolished the temperature-responsiveness of those clock genes. Thus, LUX may help to input temperature information into the A. thaliana circadian system. In addition, ELF3 association to LUX was found to be abolished at high temperatures, suggesting that temperature may play a role in recruiting EC components to their targeted promoters.
In the absence of LUX, ELF3 and ELF4 have also been found to form a complex with LUX homolog NOX (meaning “night” in Latin), also called BROTHER OF LUX ARRHYTHMO (BOA). NOX is a homologous Myb-like GARP transcription factor that binds to DNA sequences similar to LUX's binding, interacts directly with ELF4, and peaks in late evening.
Experiments involving artificial microRNA (amiRNA) methods have shown that both NOX and LUX are required to recruit the EC to the PIF4 and PIF5 promoters. There is evidence for NOX having an important role in the regulation of the plant circadian oscillator; overexpression of NOX has been found to have circadian phenotypes of long periods, as well as altered expressions of CCA1, LHY, GI (GIGANTEA), and TOC1. In particular, overexpression of NOX showed increased amplitudes of CCA1 expression. NOX likely regulates CCA1 through direct binding to the CCA1 promoter, and, conversely, CCA1 has been found to bind to the NOX promoter and inhibit NOX expression.
In contrast to LUX, amiRNA knock-outs have shown that NOX is not required for circadian rhythms, suggesting that the functionality of LUX and NOX are not completely redundant. RNAi experiments reducing NOX expression showed continuation of circadian rhythms, whereas lux null mutants are arrhythmic. Currently, more research must be done to determine how LUX and NOX differ in their contributions to the EC.
Specific studies of LUX (and ELF3) orthologous mutant alleles have identified variants in flowering and photoperiod-dependent growth.
ROC15 and ROC75
Orthologs have been found for all three members of the Arabidopsis thaliana Evening Complex, but it is currently unknown if the EC is formed in species other than A. thaliana. Two orthologs of LUX, ROC15 and ROC75, have been discovered in Chlamydomonas reinhardtii, but orthologs of ELF3 and ELF4 in C. reinhardtii have not yet been found.
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