Timeless (gene) - Function in The Circadian Clock

Function in The Circadian Clock

The timeless gene is an essential component of the molecular circadian clock in Drosophila. It acts as part of an autoregulatory feedback loop in conjunction with the period (per) gene product. Its circadian properties were noted in studies performed by Dr. Michael Rosbash's lab and Dr. Charles Weitz's lab. Both indicated that timeless protein (TIM) and period protein (PER) form a heterodimer that exhibits circadian rhythms in wild type Drosophila. Researchers in Rosbash's lab also showed that tim mRNA levels and TIM protein levels have circadian rhythms that are similar to those of the period(per) and its product.

The PER/TIM heterodimer regulates transcription of period (per) and timeless (tim) genes. First the PER/TIM heterodimers form in the cytoplasm, accumulate, and then translocate to the nucleus. The complex then blocks the positive transcription factor clock (CLK) and cycle (CYC).

As part of the circadian clock timeless is essential for entrainment to light-dark (LD) cycles. The typical period length of free-running Drosophila is 23.9 hours, requiring adaptations to the 24-hour environmental cycle. Adaptation first begins with exposure to light. This process leads to the rapid degradation of the TIM protein, allowing organisms to entrain at dawn to environmental cycles. In light-dark cycles, TIM protein level decreases rapidly in late night/early morning, followed by the similar but more gradual changes in PER protein level. TIM degradation is independent of per and its protein, and releases PER from the PER/TIM complex. This ends the PER/TIM repression of the CLK/CYC-mediated transcription of per and tim genes, allowing per and tim mRNA to be produced to restart the cycle. In some cell types, the photoreceptor protein cryptochrome (CRY) physically associates with TIM and helps regulate light-dependent degradation. CRY is activated by blue light, which binds to TIM and tags it for degradation.

This mechanism allows entrainment of flies to environmental light cues. When Drosophila receive light inputs in the early subjective night, light-induced TIM degradation causes a delay in TIM accumulation, which creates a phase delay. When light inputs are received in the late subjective night, a light pulse causes TIM degradation to occur earlier than under normal conditions, leading to a phase advance.

In Drosophila, the negative factors PER/TIM, as well as the positive factors CLK/CYC, are eventually degraded by a casein kinase-mediated phosphorylation cycle, allowing fluctuations in gene expression according to environmental cues. These proteins mediate the oscillating expression of the transcription factor VRILLE (VRI), which is required for behavioral rhythmicity, per and tim expression, and accumulation of PDF (pigment-dispersing factor).