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''Cycle'' (''cyc'') is a gene in ''Drosophila melanogaster'' that encodes the CYCLE protein. The ''Cycle'' gene is expressed in a variety of cell types in a circadian manner. It is involved in controlling both the sleep-wake cycle and circadian regulation of gene expression by promoting transcription in a negative feedback mechanism. The ''cyc'' gene is located on the left arm of chromosome 3 and codes for a transcription factor containing a basic helix-loop-helix (bHLH) domain and a PAS domain. The ''cyc'' gene is 2.17 kb in size, made up of 5 coding exons totaling 1,625 base pairs coding for 413 aminos acid residues, and there are currently 19 known alleles.〔(【引用サイトリンク】url=http://flybase.org/reports/FBgn0023094.html )|accessdate=10 April 2013〕 Orthologs performing the same function in other species include ARNTL and BMAL1. ==Function== ''Cyc'' is primarily known for its role in the genetic transcription-translation feedback loop that generates circadian rhythms in ''Drosophila''. In the cell nucleus, the CYCLE protein (CYC) forms a heterodimer with a second bHLH-PAS protein, CLOCK (CLK). This CYC-CLK protein complex binds to E-box elements in promoter regions of the genes period and timeless, functioning as a transcription factor in the translation of the proteins PER and TIM. After PER and TIM proteins accumulate in the cytoplasm and bind together, the PER-TIM complex translocates to the nucleus and interferes with the CYC-CLK protein complex function to inhibit its activation of transcription. Thus, CLK and CYC act as positive factors and PER and TIM as negative factors. CYC also plays a role in the post-translational regulation of CLK in the cytoplasm. These four proteins of the feedback loop are later degraded by a casein kinase-mediated phosphorylation cycle, allowing fluctuations in gene expression according to environmental cues. The Howard Hughes Medical Institute has an educational video detailing the (transcription-translation feedback loop ). ''Cyc'' is involved with the genetic basis of other behaviors that relate to circadian processes, such as Drosophila’s responses to starvation and sleep deprivation, and regulation of rest and life span. Starvation in Drosophila potently suppresses sleep, suggesting that the homeostatically regulated behaviors of feeding and sleep are integrated in flies. ''Clk'' and ''cyc'' act during starvation to modulate the conflict of whether flies sleep or search for food, thus playing a critical role for proper sleep suppression during starvation. The role of ''cyc'' in response to sleep deprivation involves expression of heat-shock genes, but is not fully understood. Its role in regulation of rest and longevity in Drosophila is also not well understood, but evidence indicates that this role displays gender dimorphism. ''Cyc'' is expressed constitutively (continuously) in Drosophila cells〔 and is present in naive Drosophila tissue culture cells, unlike ''clk'', ''per'', or ''tim''. Regulation thus occurs primarily through the negative feedback by the PER-TIM protein complex in the transcription-translation feedback loop described above. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cycle (gene)」の詳細全文を読む スポンサード リンク
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