The Circadian clock is an endogenous molecular feedback loop that maintains about a 24 hour period over a range of physiologically relevant temperatures and can be synchronized to the exact solar day by light, temperature and other environmental cues. A critical role of the circadian clock is to partition, or phase specific biological activities such as metabolism, and transcription to occur at distinct times over the day. Utilizing an in vivo luciferase assay in Arabidopsis thaliana I defined a phase-specific cis-acting element, characterized the extent of circadian transcriptional control by enhancer trapping, and established the role of temperature and light in setting circadian phase. In the later study I was also able to use temperature to separate two molecular oscillators in the same cell. Furthermore, I utilized the luciferase assay to characterized the phasing defects of the out of phase 1(oop1) mutant, which is a novel allele of the red light photoreceptor PHYTOCHROME B. This work established a link between photoperception and accurate circadian phasing. Finally, I demonstrated that there exists a great deal of natural variation in circadian period, amplitude and phase, mapped quantitative trait loci (QTL), and established that phase is an integral aspect of period.
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