Virtually all eukaryotic organisms appropriately examined have been shown to possess the capacity for endogenous temporal control and organization known as a circadian rhythm. The cellular machinery responsible for generating rhythms is collectively known as the biological clock. A healthy circadian clock underlies both physical and mental health. Because of the ubiquity of its influence on human mental and physiological processes - from circadian changes in basic human physiology to the clear involvement of rhythms in work/rest cycles and sleep - understanding the clock is basic to prevention and treatment of many physical and mental illnesses, from metabolic disorders to sleep/wake dysfunction and cancer. Our specific aims are designed to further our understanding of the means through which this clock that regulates cell behavior is organized. Specific Aim #1 builds upon our understanding of the interplay between structure and function in core clock components. New data are inconsistent with some existing models and predicted roles for clock proteins. We will identify updated roles, interactions, and structures, as well as probing how time-of-day-specific clock-controlled phosphorylation guides essential interactions and activities of clock components. Specific Aim#2 re-examines the overarching regulatory architecture of the eukaryotic circadian oscillator in light of new data that challenge a major precept of current theory, the essential role of protein turnover. This also prompts a closer look at the role of phosphorylation in temperature compensation and the kinases that bring this about. Specific Aim #3 builds upon our strong grounding in genetics and genomics as well as ongoing work that has identified previously unstudied genes whose mutation alters period length by as much 18 hours. Examination of the bases of these effects will take us into aspects metabolism and gene expression heretofore unexplored in terms of circadian biology. Our long term goals are to describe, in the language of genetics and biochemistry, the feedback cycle comprising the circadian clock, how this cycle is synchronized with the environment, and how time information generated by the feedback cycle is used to regulate the behavior of cells and organisms. These projects are complementary and mutually enriching in that they rely on genetic and molecular techniques to dissect, and ultimately to understand, the organization of cells as a function of time.