Timekeeping in bacteria: the cyanobacterial circadian clock

The prokaryotes known as cyanobacteria possess an endogenous 24 h biological (circadian) clock that provides temporal coordination for physiological processes. Although the cyanobacterial clock has the same fundamental properties as circadian clocks in eukaryotes, its components are nonhomologous to those of animals, plants or fungi. Moreover, its mechanism is likely to be very different from that depicted in eukaryotic clock models. The picture that is emerging for the timing mechanism in cyanobacteria is of a multiprotein, multimeric, molecular machine composed of proteins whose domains exhibit twists on common themes. Signal transduction into and out of the clock core appears to occur via histidine protein kinase-based phosphorylation relays.