Further clues into the regulation of circadian clocks are the subject of two reports. In plants and insects, cryptochromes (CRYs) are activated by light, and in the fruit fly Drosophila, CRY blocks the negative feedback action of the PER-TIM complex. Griffin et al. show that CRY1 and CRY2 play a central role in the mammal clock, but in a light-independent fashion--they appear to regulate transcriptional cycling of Per1 by contacting both the activator and its feedback inhibitors. In Drosophila, three of the critical clock genes, period (per), timeless (tim), and Drosophila Clock (dClk), are expressed rhythmically. In their study of the cycling of dClk, Glossop et al. have found that the molecular clock in Drosophila is composed of two interlocked negative feedback loops--the per-tim loop, which is activated by the dCLK and CYCLE proteins and repressed by PER-TIM, and the dClk loop, in which these proteins exert the opposite effect.
Griffin Jr., E.A., Staknis, D., and Weitz, C.J. (1999) Light-independent role of CRY1 and light-independent role of CRY1 and light-iIndependent role of CRY1 and CRY2 in the mammalian circadian clock. Science 286: 768 - 771. [Abstract] [Full Text]