In addition to light, temperature can affect circadian cycles of behavioral activity. Kaushik et al. found that fruit flies undergo phase shifts in their circadian behavior in response to pulses of elevated temperature--37°C, but not 30°C--during the early part of the "night" (dark period). Light pulses during the dark period caused both delays and advances in the circadian cycle depending on when the light was applied. Coimmunoprecipitation experiments revealed that in head extracts from the flies, the 37°C heat pulse or light promoted an interaction between the cryptochrome CRY, which is the light sensor for the circadian clock, and the transcriptional regulators TIM (Timeless) and PER (Period), which form a heterodimer and are part of the feedback loop that controls the circadian cycle (see the Drosophila Circadian Pathway). The 37°C heat pulse was only effective at promoting the interaction or delaying the circadian cycle when applied early in the dark period [Zeitgeber time 15 (ZT15), where ZT0 is the number of hours elapsed since dawn]. The heat- or light-induced phase shifts were lost in loss-of-function cryb flies, confirming that CRY is important for responding to either heat or light. A PER mutant called perL exhibited more extreme phase shift changes in response to heat and light, including both delays and advances in the cycle depending on when the pulse of heat or light was applied, and also exhibited phase shift changes in response to 30°C temperature pulses. The interaction between CRY and PER and TIM detected by coimmunoprecipitation was promoted even when the mutant flies were exposed to the 30°C heat pulse. The authors suggest that the PerL mutation or heat trigger a conformational change in TIM that promotes its interaction with CRY, thereby altering the circadian cycle.
R. Kaushik, P. Nawathean, A. Busza, A. Murad, P. Emery, M. Rosbash, PER-TIM interactions with the photoreceptor cryptochrome mediate circadian temperature responses in Drosophila. PLoS Biol. 5, e146 (2007). [PubMed]