Van Gelder R N, Bae H, Palazzolo M J, Krasnow M A
Department of Biochemistry, Stanford University School of Medicine, California 94305, USA.
Curr Biol. 1995 Dec 1;5(12):1424-36. doi: 10.1016/s0960-9822(95)00280-6.
Although mRNAs expressed with a circadian rhythm have been isolated from many species, the extent and character of circadianly regulated gene expression is unknown for any animal. In Drosophila melanogaster, only the period (per) gene, an essential component of the circadian pacemaker, is known to show rhythmic mRNA expression. Recent work suggests that the encoded Per protein controls its own transcription by an autoregulatory feedback loop. Per might also control the rhythmic expression of other genes to generate circadian behavior and physiology. The goals of this work were to evaluate the extent and character of circadian control of gene expression in Drosophila, and to identify genes dependent on per for circadian expression.
A large collection of anonymous, independent cDNA clones was used to screen for transcripts that are rhythmically expressed in the fly head. 20 of the 261 clones tested detected mRNAs with a greater than two-fold daily change in abundance. Three mRNAs were maximally expressed in the morning, whereas 17 mRNAs were most abundant in the evening--when per mRNA is also maximally expressed (but when the flies are inactive). Further analysis of the three 'morning' cDNAs showed that each has a unique dependence on the presence of a light-dark cycle, on timed feeding, and on the function of the per gene for its oscillation. These dependencies were different from those determined for per and for a novel 'evening' gene. Sequence analysis indicated that all but one of the 20 cDNAs identified previously uncloned genes.
Diurnal control of gene expression is a significant but limited phenomenon in the fly head, which involves many uncharacterized genes. Diurnal control is mediated by multiple endogenous and exogenous mechanisms, even at the level of individual genes. A subset of circadianly expressed genes are predominantly or exclusively dependent on per for their rhythmic expression. The per gene can therefore influence the expression of genes other than itself, but for many rhythmically expressed genes, per functions in conjunction with external inputs to control their daily expression patterns.
尽管已从许多物种中分离出具有昼夜节律表达的mRNA，但对于任何动物而言，昼夜节律调节的基因表达程度和特征仍不清楚。在黑腹果蝇中，仅已知昼夜节律起搏器的必需成分周期（per）基因表现出节律性mRNA表达。最近的研究表明，编码的Per蛋白通过自调节反馈环控制其自身的转录。Per也可能控制其他基因的节律性表达，以产生昼夜行为和生理功能。这项工作的目标是评估果蝇中基因表达的昼夜控制程度和特征，并鉴定昼夜表达依赖于per的基因。
使用大量匿名、独立的cDNA克隆来筛选在蝇头中有节律表达的转录本。在测试的261个克隆中，有20个检测到丰度每日变化超过两倍的mRNA。三种mRNA在早晨表达最高，而17种mRNA在晚上最丰富——此时per mRNA也表达最高（但果蝇不活动）。对这三种“早晨”cDNA的进一步分析表明，每种cDNA在其振荡对明暗周期的存在、定时进食以及per基因功能的依赖性方面都具有独特性。这些依赖性与针对per和一个新的“晚上”基因所确定的依赖性不同。序列分析表明，20个cDNA中除一个外均鉴定出先前未克隆的基因。
基因表达的昼夜控制在蝇头中是一个显著但有限的现象，涉及许多未表征的基因。昼夜控制由多种内源性和外源性机制介导，甚至在单个基因水平也是如此。一部分昼夜节律表达的基因在其节律性表达上主要或完全依赖于per。因此，per基因可以影响除自身以外的其他基因的表达，但对于许多节律性表达的基因，per与外部输入共同作用以控制它们的每日表达模式。
