Circadian rhythms in Synechococcus elongatus PCC 7942: insights into the regulatory mechanisms of the cyanobacterial clock system
Circadian rhythms of behavior have been well characterized in organisms including mammals, plants, insects, fungi, and photosynthetic bacteria. Cyanobacteria, such as the unicellular Synechococcus elongatus PCC 7942, display near 24-h circadian rhythms of gene expression. These rhythms persist in the absence of external cues, can be reset by the same stimuli to which they entrain, and are relatively insensitive to changes in ambient temperature within their physiological range. Key components have been identified as belonging to the central oscillator that comprises the timekeeping units, output pathways that relay temporal information to clock-controlled processes, and input pathways that synchronize the oscillator with local time. The emerging model of the cyanobacterial clock depicts the internal timekeeping elements KaiA, KaiB, and KaiC interacting with one another to form a large, multimeric complex that assembles and disassembles over the course of a day. Information is sent into and out of the oscillator via signal transduction pathways that include proteins involved in bacterial twocomponent systems. The research presented in this dissertation explores the regulatory mechanisms that exist at each level of the clock system. New components were identified that interact with an important protein in the input pathway; these new players are involved in clock-associated phenomena, such as resetting the internal oscillation to external stimuli and maintaining proper circadian periodicity, as well as the process of cell division. The model formerly associated with the temporal, transcriptional regulation of the kai genes was redefined to reflect the unique properties of the prokaryotic oscillator. The differential output of the clock was examined by studying the circadian regulation of the psbA gene family. Overall, these data provide insight into the complex molecular events that occur to create a circadian timing circuit in S. elongatus.