Seeing the light

Despite the enormous progress made in recent years towards unravelling the autoregulatory transcriptional feedback loops that underlie circadian rhythms in organisms ranging from cyanobacteria to man, many mysteries remain.

As delegates at the Novartis Foundation Symposium on 'Molecular clocks and light signalling' in London heard this week, not least among the unknowns for many organisms is the identity of the photopigment that transmits signals about light and dark in such a way that each organisms' internal circadian clock becomes synchronized with night and day.

In mammals, evidence from mice lacking rod and cone photoreceptors crystallised the view that there must be a photo pigment other than rhodospin that signals to the circadian clock. Prime candidates include the cryptochromes, homologs of blue-light receptors in Arabidopsis and circadian photoreceptors in the Drosophila brain, and melanopsin. The latter is a member of the same family as rhodospin but is found in retinal cells...