What the data showed was that about 10 percent of the genes cycled, but most showed little overlap from tissue to tissue. This type of cycling has to do with local physiology -- for instance, the liver's expression of certain enzymes at certain times of the day. What they were really interested in were the overlapping cyclers -- those genes that cycled in all tissue types. These, they reckoned, would be part of the master clock.
They found 50 genes that cycled at the same time throughout the day across all the various tissues, and they speculated that this collection would include both known and unknown circadian rhythm genes. Indeed, known circadian genes were among the 50, but there were dozens of other cycling genes that had not been previously identified as clock genes.
The scientists speculated that some of these other genes may be part of the mammalian circadian clock, and they reasoned that if they were, they might interact with some of the known clock genes. So they designed an experiment to see if expressing the new genes in a cell could change the expression of known clock genes. To do this, they used a biochemical assay to detect if any of these genes had the ability to control transcription -- that crucial first step in the expression of a gene.
They discovered that a family of genes called the retinoic acid receptor-related orphan receptor-a (Rora) cycled and had the ability to control transcription. These are so named because they are similar in amino acid sequence to the retinoic acid receptor genes, although they do not have the same function and do not bind to retinoic acid. (They are called orphan receptors because it is not known what activates
Contact: Keith McKeown
Scripps Research Institute