Clock-gated photic stimulation of timeless expression at cold temperatures and seasonal adaptation in Drosophila

Abstract Numerous lines of evidence indicate that the initial photoresponse of the circadian clock in Drosophila melanogaster is the light-induced degradation of TIMELESS (TIM). This posttranslational mechanism is in sharp contrast to the well-characterized pacemakers in mammals and Neurospora, where light evokes rapid changes in the transcriptional profiles of 1 or more clock genes. The authors show that light has novel effects on D. melanogaster circadian pacemakers, acutely stimulating the expression of tim at cold but not warm temperatures. This pho-toinduction occurs in flies defective for the classic visual phototransduction pathway or the circadian-relevant photoreceptor CRYPTOCHROME (CRY). Cold-specific stimulation of tim RNA abundance is regulated at the transcriptional level, and although numerous lines of evidence indicate that period (per) and tim expres-sion are activated by the same mechanism, light has no measurable acute effect on per mRNA abundance. Moreover, light-induced increases in the levels of tim RNA are abolished or greatly reduced in the absence of functional CLOCK (CLK) or CYCLE (CYC) but not PER or TIM. These findings add to a growing number o