Cells of diverse organisms, from cyanobacteria to humans, execute temporal programs of physiological processes that are driven by circadian oscillators. For example, the Synechococcus elongatus circadian clock regulates global patterns of gene expression, the timing of cell division, and compaction of the chromosome. Our research uses S. elongatus as a model to understand how a cell keeps track of time, executes activities according to a temporal program, and synchronizes the internal clock with the external solar cycle. The components of the circadian oscillator are known (proteins KaiA, KaiB, and KaiC), their structures have been solved, and the circadian rhythm in phosphorylation of KaiC can be reconstituted in vitro. Our recent work has focused on the mechanisms that coordinate the clock with metabolism to keep circadian rhythms aligned properly with the solar day. By sampling two metabolic measures of photosynthetic activity, the S. elongatus circadian oscillator can sense both the onset and duration of darkness to set the clock. We are also investigating the biochemical states of KaiC that stimulate the major output pathway that controls gene expression. This work has shown that a specific phospho-state of KaiC affects both dawn- and dusk-peaking promoter expression, but with opposite outcomes regarding stimulation and repression.
