The long term goal of this proposal is to understand how circadian clock are built into eukaryotic cells. Circadian clocks are known to regulate many essential cellular processes widely distributed across biology. In higher plants, the clock is involved in regulating diverse processes ranging from photosynthesis to control of flowering time. We have chosen Arabidopsis as a model organism and have identified several clock genes from mutational analysis. The regulation of these clock genes has defined a molecular feedback loop which forms the basis for models of the clock in higher plants. The experiments in this proposal aim to build on the current clock model by continuing to identify clock factors and determine their position within the circadian system. The circadian clock in higher plants regulates a vast number of genes and utilizing genomics approaches to identify clock-regulated genes has provided clock biologists with a powerful tool. This proposal will extend our initial experiments with Circadian Genomics by analyzing the Arabidopsis full genome chip now commercially available. By surveying genes that are regulated by the clock, we will identify novel clock associated factors and define their positions within the circadian system. In addition, recent results have identified key proteins involved in the photoperiodic control of flowering time. Experiments in this proposal will further dissect the molecular communication between the clock and flowering time. Given the ubiquity of circadian-regulated physiology, characterization of circadian systems in model organisms will impact on understanding of the pacemaker mechanisms and malfunctions associated with known features of human well-being.