Circadian Programmes in Cyanobacteria

Abstract

Cyanobacteria are prokaryotes that express circadian (daily) rhythms. In rhythmic environments, the fitness of cyanobacteria is improved when this clock is operational and when its circadian period is similar to the period of the environmental cycle. In cyanobacteria, three key proteins (KaiA, KaiB and KaiC) form a core molecular clockwork that orchestrates global gene expression by modulating chromosomal topology. The three‐dimensional structures of these proteins have been determined. KaiA, KaiB and KaiC form a multiprotein nanomachine that can reconstitute a circadian oscillator in vitro , and this oscillator appears to function as a post‐translational oscillator (PTO) in vivo . Because this PTO regulates rhythmic transcription and translation, the entire circadian system comprises both the PTO and a transcriptional/translational feedback loop. Models of the complete in vivo system have important implications for our understanding of circadian clocks in higher organisms, including mammals. Key Concepts: Prokaryotic cyanobacteria have a circadian timekeeping system that enhances fitness. These cells exhibit pervasive circadian regulation of gene expression, possibly mediated by cyclic changes of chromosomal topology. A circadian rhythm of the phosphorylation of the central clock protein KaiC can be reconstituted in vitro with three proteins derived from cyanobacteria (KaiA, KaiB and KaiC) and ATP. KaiA, KaiB and KaiC are the only circadian clock proteins for which the 3‐D structure of full‐length proteins is known. Structural, biochemical and biophysical methods have been used to study the mechanism by which KaiC is rhythmically phosphorylated and dephosphorylated. Mathematical modelling that has been applied to the in vitro and in vivo systems indicate the existence of a core biochemical post‐translational oscillator (PTO) that controls a larger transcription/translation feedback loop (TTFL).