Calvi B R, Lilly M A, Spradling A C
Howard Hughes Medical Institute Research Laboratories, Carnegie Institution of Washington, Baltimore, Maryland 21210, USA.
Genes Dev. 1998 Mar 1;12(5):734-44. doi: 10.1101/gad.12.5.734.
Over-replication of two clusters of chorion genes in Drosophila ovarian follicle cells is essential for rapid eggshell biosynthesis. The relationship of this amplification to the follicle cell cycles has remained unclear. To investigate the regulation of amplification, we developed a technique to detect amplifying chorion genes in individual follicle cells using BrdU incorporation and FISH. Amplification occurs in two developmental phases. One of the gene clusters begins to amplify periodically during S phases of follicle cell endocycles. Subsequently, after endocycles have ceased, both clusters amplify continuously during the remainder of oogenesis. In contrast to the early phase, late amplification commences synchronously among follicle cells. The pattern of Cyclin E expression mirrors these two phases. We present evidence that Cyclin E is required positively for amplification. We suggest that Cyclin E also acts negatively to inhibit refiring of most origins within a cycle, and that specific factors at chorion origins allow them to escape this negative rereplication control. Our findings suggest that chorion amplification is a model for understanding metazoan replicons and the controls that restrict replication to once per cell cycle.
果蝇卵巢滤泡细胞中两个绒毛膜基因簇的过度复制对于快速合成卵壳至关重要。这种扩增与滤泡细胞周期的关系尚不清楚。为了研究扩增的调控机制，我们开发了一种利用BrdU掺入和FISH检测单个滤泡细胞中扩增的绒毛膜基因的技术。扩增发生在两个发育阶段。其中一个基因簇在滤泡细胞内循环的S期开始周期性扩增。随后，在内循环停止后，两个基因簇在卵子发生的剩余阶段持续扩增。与早期阶段不同，晚期扩增在滤泡细胞之间同步开始。细胞周期蛋白E的表达模式反映了这两个阶段。我们提供的证据表明，细胞周期蛋白E对扩增起正向作用。我们认为，细胞周期蛋白E也起负向作用，抑制一个周期内大多数起始位点的重新激活，并且绒毛膜起始位点的特定因子使它们能够逃避这种负向的再复制控制。我们的研究结果表明，绒毛膜扩增是理解后生动物复制子以及将复制限制在每个细胞周期一次的控制机制的一个模型。
