Molecular Mechanisms of Clock-Controlled Proteins in Phytoflagellates

Circadian rhythms in microalgae

Circadian rhythms have been described in a variety of microalgae. In each group, some model organisms arose and most detailed studies have been done with them. They include the cyanobacterium ("blue-green alga") Synechococcus and eukaryotic microalgae Gonyaulax polyedra (Dinophyta), Chlamydomonas reinhardtii (Chlorophyta), and Euglena gracilis (Euglenophyta). This review focuses on recent approaches to depict molecular components of the circadian system and the mechanisms of regulation in these organisms. In Synechococcus, the identification of the kailocus, which represents a central part of its oscillatory system, is discussed, as well as diverse approaches based on a luminescent reporter gene, which is driven by a clock-controlled cyanobacterial promoter. In eukaryotic microalgae, the diversity of genes/proteins that are controlled by the circadian clock is described and the kind of regulation (transcriptional and translational control) is emphasized. The role and function of conserved clock-controlled RNA-binding proteins such as CCTR from Gonyaulaxor Chlamy 1 from Chlamydomonas are discussed.

Endogenous timekeepers in photosynthetic organisms

Circadian and photoperiodic timing mechanisms were first described in photosynthetic organisms. These organisms depend upon sunlight for their energy, so adaptation to daily and seasonal fluctuations in light must have generated a strong selective pressure. Studies of the endogenous timekeepers of photosynthetic organisms provide evidence for both a fitness advantage and for selective pressures involved in early evolution of circadian clocks. Photoperiodic timing mechanisms in plants appear to use their circadian timers as the ruler by which the day/night length is measured. As in animals, the overall clock system in plants appears to be complex; the system includes multiple oscillators, several input pathways, and a myriad of outputs. Genes have now been isolated from plants that are likely to encode components of the central clockwork or at least that act very close to the central mechanism. Genetic and biochemical analyses of the central clockwork of a photosynthetic organism are most highly advanced in cyanobacteria, where a cluster of clock genes and interacting factors have been characterized.