Project Summary/Abstract Circadian clocks are intracellular enzymatic systems that provide a biochemical representation of local time with profound consequences to health across diverse organisms. Unlike most enzymes, biological clocks need to be insensitive to a range of physiological temperatures and cellular energy levels so that organisms can anticipate dawn and dusk reliably. However, the mechanisms by which circadian clocks in any organism achieve insensitivity to temperature (i.e., temperature compensation) and cellular energy levels (i.e., metabolic compensation) are far from understood. Therefore, the overall vision of the LiWang lab for the next five years is to elucidate the mechanisms of temperature compensation and metabolic compensation in the circadian clock of cyanobacteria. Over the past 20 years, we have made many impactful discoveries on mechanism of the cyanobacterial clock and developed innovative methodologies and tools along the way. Thus, we are very well positioned to succeed at filling critical gaps in knowledge in the field of biological timekeeping. A major expected outcome of the work proposed here is a detailed cause-and-effect model linking clock protein behavior and interactions to temperature and metabolic compensation phenotypes in vivo.