CELL-CYCLE CONTROL OF CHORION GENE AMPLIFICATION

Over-replication of two clusters of chorion genes in Drosophila ovaria n follicle cells is essential for rapid eggshell biosynthesis. The rel ationship of this amplification to the follicle cell cycles has remain ed unclear. To investigate the regulation of amplification, we develop ed a technique to detect amplifying chorion genes in individual follic le cells using BrdU incorporation and PISH. Amplification occurs in tw o developmental phases. One of the gene clusters begins to amplify per iodically during S phases of follicle cell endocycles. Subsequently, a fter endocycles have ceased, both clusters amplify continuously during the remainder of oogenesis. In contrast to the early phase, late ampl ification commences synchronously among follicle cells. The pattern of Cyclin E expression mirrors these two phases. We present evidence tha t Cyclin E is required positively for amplification We suggest that Cy clin E also acts negatively to inhibit refiring of most origins within a cycle, and that specific factors at chorion origins allow them to e scape this negative rereplication control. Our findings suggest that c horion amplification is a model for understanding metazoan replicons a nd the controls that restrict replication to once per cell cycle.