How a circadian clock adapts to seasonal decreases in temperature and day length

We show that a thermosensitive splicing event in the 3' untranslated region of the mRNA from the period (per) gene plays an important role in how a ci rcadian clock in Drosophila adapts to seasonally cold days (low temperature s and short day lengths). The enhanced splicing of this intron at low tempe ratures advances the steady state phases of the per mRNA and protein cycles , events that significantly contribute to the preferential daytime activity of flies on cold days. Because the accumulation of PER is also dependent o n the photosensitive TIMELESS (TIM) protein, long photoperiods partially co unteract the cold-induced advances in the oscillatory mechanism by delaying the daily increases in the levels of TIM. Our findings also indicate that there is a temperature-dependent switch in the molecular logic governing cy cles in per mRNA levels.