Circadian Rhythms in Cyanobacteria

Abstract

Life on earth is subject to daily and predictable fluctuations in light intensity, temperature, and humidity created by rotation of the earth. Circadian rhythms, generated by a circadian clock, control temporal programs of cellular physiology to facilitate adaptation to daily environmental changes. Circadian rhythms are nearly ubiquitous and are found in both prokaryotic and eukaryotic organisms. Here we introduce the molecular mechanism of the circadian clock in the model cyanobacterium Synechococcus elongatus PCC 7942. We review the current understanding of the cyanobacterial clock, emphasizing recent work that has generated a more comprehensive understanding of how the circadian oscillator becomes synchronized with the external environment and how information from the oscillator is transmitted to generate rhythms of biological activity. These results have changed how we think about the clock, shifting away from a linear model to one in which the clock is viewed as an interactive network of multifunctional components that are integrated into the context of the cell in order to pace and reset the oscillator. We conclude with a discussion of how this basic timekeeping mechanism differs in other cyanobacterial species and how information gleaned from work in cyanobacteria can be translated to understanding rhythmic phenomena in other prokaryotic systems.

FIG 1

Mechanism of the cyanobacterial circadian oscillator. KaiC is a hexameric protein that comes together to form a “double doughnut” shape comprised of CI and CII rings with A-loop segments extending from the C terminus in the CII ring. KaiC in the nonphosphorylated state (ST) (far left) can interact with KaiA. KaiA, a dimer, binds to and stabilizes the A-loop segment in the exposed state, promoting KaiC autophosphorylation during the day. Red circles enclosing “S” and “T” represent phosphorylation of S431 and T432 residues, respectively, located in the CII ring. T432 becomes phosphorylated first (SpT), followed by S431, resulting in the doubly phosphorylated species (pSpT). Phosphorylation of S431 induces the stacking of the CII ring onto the CI ring. Ring-ring stacking exposes a binding site for KaiB (B-loop) on the CI ring. In order to for KaiB to interact with KaiC, both the B-loop must be exposed and KaiB must rearrange its tertiary fold from a ground-state fold found in tetramer and dimer species (orange circles) to a monomeric fold-switched form, fs-KaiB (orange diamond). Once both are achieved, fold-switched KaiB can bind to KaiC. fs-KaiB also interacts with KaiA, sequestering KaiA away from the A-loops on the CII domain. This action promotes burial of the A-loops into the hexamer and KaiC's autophosphatase activity during the nighttime portion of the cycle. Dephosphorylation occurs first on T432, resulting in pST species, followed by S431, resulting in a nonphosphorylated species (ST).

FIG 2

Timeline of circadian events. Peak timing for various events that occur over the circadian cycle is depicted both numerically and graphically (dots). All data except for RpaB phosphorylation status were collected from experiments performed in constant light.

FIG 3

Model for how the circadian network functions over time and within a cell. Two cells are displayed side by side showing the progression of the oscillator over time (indicated by shading from sun to moon at the top and numerically at the bottom). During the day (left panel), KaiA and KaiC are found primarily in the cytosol. KaiA interacts with KaiC, promoting KaiC phosphorylation during the illuminated portion of the cycle. Phosphorylation events are indicated by a red dot. CikA is localized to the poles of cells. Quinones mobile in the thylakoid membranes are reduced during the day (red “Q”). SasA is capable of interacting with KaiC, promoting autophosphorylation of SasA and phosphotransfer to RpaA. RpaA-P accumulates over the course of the day, peaking at the day-night transition or ∼12 h after lights on. RpaA-P represses the expression of class 2 (dawn-peaking) genes and activates class 1 (dusk-peaking) genes. Environmentally controlled RpaB can inhibit the phosphorylation of RpaA. Chromosomes (gray) are in a noncompacted state. At night or during the subjective night (right panel), the quinone pool becomes transiently oxidized. KaiA, KaiC, and CikA are all colocalized to the cell poles where CikA and KaiA can interact with oxidized quinones (blue “Q”). Fold-switched KaiB competes with SasA for binding to KaiC and sequesters KaiA away from the A-loop, promoting KaiC dephosphorylation through the night. CikA competes with KaiA for binding to fold-switched KaiB. The KaiBC complex promotes CikA's phosphatase activity toward RpaA. At night, chromosomes (gray) are in a compacted state.