Figure 4

Molecular mechanisms of the circadian clock and effects of clock gene RNAi on the circadian clock in Riptortus pedestris . (A) A general model of the insect circadian clock mechanism. Positive elements of CYC and CLK form a heterodimer and activate transcription of clock genes per, tim, and cry-m and many other genes called clock-controlled genes (ccg). Their protein products are synthesized in the cytoplasm and PER, TIM and CRY-m form a complex. In Drosophila melanogaster, only PER and TIM form a heterodimer. The complex enters the nucleus and acts as a negative element that represses CYC/CLK transcriptional activity. Reduction of ccg transcript levels and consequent reduction of CCG protein levels lead to a decrease of repressive regulation of CYC/CLK by PER/TIM/CRY-m, and therefore CYC/CLK-mediated transcription increases again. These phases in which ccg transcription is activated or repressed are repeated in approximately 24 h. CRY-d causes degradation of TIM in a light-dependent manner. (B) When negative elements PER and CRY-m are eliminated by RNAi in R. pedestris, the circadian clock remains at the phase in which ccg transcription is activated [129]. (C) When positive elements CYC and CLK are eliminated, the circadian clock remains at the phase in which ccg transcription does not occur [130]. Note that the absence of transcriptional activity would also decrease the protein levels of negative elements.