Mechanistic Aspects of the Cyanobacterial Circadian Clock

Circadian clocks are intracellular systems that provide an internal representation of time to regulate metabolism in preparation for day and night. A variety of mechanisms arose throughout the evolutionary tree as an adaptation to predictable daily swings in ambient light and temperature. In this chapter we will focus exclusively on the circadian clock shared by the cyanobacteria Synechococcus elongatus and Thermosynechococcus elongatus, with a noncomprehensive scope that emphasizes mechanism. As will hopefully become apparent here, the cyanobacterial system is valuable to the broader circadian clocks field because it yields conceptual insights into biological timekeeping at a level of detail higher and more comprehensive than those achieved so far in other systems. Major takeaways of this chapter include the following: (1) coordination of moving clock components is achieved through long-range allostery mediated by changes in structure and dynamics such that the clock runs clockwise and not counterclockwise, and (2) the fold-switching behavior of a clock protein underpins circadian rhythms of gene expression.