The Gonyaulax clock at 50: translational control of circadian expression.

The unicellular circadian clock of Gonyaulax polyedra (now renamed Lingulodinium polyedrum) has provided important insights concerning circadian rhythmicity. Many, perhaps most, of its key systems are circadian-controlled, ranging from bioluminescence and photosynthesis to motility, cell division, and the synthesis of many proteins, favoring the "master clock" concept. But different rhythms may have different free-running periods and different phase angles under different T cycles, observations not easily accommodated in a single oscillator model. Gonyaulax has a feature significantly different from that of other known systems, namely, that clock control of protein synthesis occurs at the translational level. With one mRNA, this involves a protein binding to a 22-nucleotide region in the 3'-untranslated region (3'UTR), but no similar regions have been found in other mRNAs. Pulses of protein synthesis inhibitors cause phase shifts, whereas inhibitors of protein phosphorylation administered chronically cause period changes. In Gonyaulax and other systems, low temperature results in arrhythmicity. A return to a permissive temperature results in a reinitiation of the rhythm, with the phase established by the time of increase, similar to the effect of bright light. Evidence for cellular communication via substance(s) in the medium has been obtained.