How Drosophila Reacts to circadian rhythm? If phosphate group is added to PER protein it fastens it's degradation. Phosphorylation is done by double time (here triangles). But if PER protein is connected with TIM protein, complex is resistant to phosphorylation. As TIM/PER accumulates, and enters nucleus suppressing expression of per and tim genes. As sun rises up, it activates cryptochrome (orange diamond inactive and circle active). Cryptochrome degrades TIM proteins, after PER proteins are phosphorylated and degraded. After degradation of TIM/PER proteins, tim and per genes become activated, once again.