November 25, 1998-Several teams of scientists have found what may be the missing molecular link between sunlight and the circadian clocks in both mammals and fruit flies.
Circadian rhythms, the patterns of activity that occur on a 24-hour cycle, are important biological regulators in virtually every living creature. In mammals, the internal circadian clock resides in the brain, and sunlight is the cue that rewinds this clock daily. Researchers have now found a biochemical pathway that senses blue light, and thereby connects the sun to molecular components of the circadian clock, says Joseph Takahashi, an HHMI investigator at Northwestern University.
In an article published in the November 20, 1998, issue of Science, Takahashi and Aziz Sancar of the University of North Carolina School of Medicine in Chapel Hill report that the protein cryptochrome 2 serves as a transducer by which light drives the molecular machinery that generates circadian rhythm. Cryptochromes are light-reactive pigments found in the eye and in plants.
Cryptochrome proteins 1 and 2 (CRY1 and CRY2), which were first discovered in plants, trigger plant growth by responding to light in the blue to ultraviolet part of the spectrum. Another article in the same issue of Science explains how CRY1 and CRY 2 help regulate circadian rhythm in plants.
In the May 26, 1998, issue of Proceedings of the National Academy of Sciences,
Yasuhide Miyamoto and Sancar showed that cells in the mouse and human retina
express genes similar to those that code for plant cryptochromes. Takahashi,
Sancar and their colleagues sought to find whether these genes were part of the
eye's circadian light-sensing apparatus. There was ample circumstantial
evidence: They knew that the same retinal cells that contain CRY1 and CRY2 are
connected directly to the suprachiasmatic nucleus (SCN), a group of neurons deep
in the mouse brain. The SCN has long been thought to be a crucial circadian
pacemaker in mammals. They
Contact: Jim Keeley
Howard Hughes Medical Institute