ORC localization in Drosophila follicle cells and the effects of mutationsin dE2F and dDP

We isolated mutations in Drosophila E2F and DP that affect chorion gene amp lification and ORC2 localization in the follicle cells. In the follicle cel ls of the ovary, the ORC2 protein is localized throughout the follicle cell nuclei when they are undergoing polyploid genomic replication, and its lev els appear constant in both S and G phases. In contrast, when genomic repli cation ceases and specific regions amplify, ORC2 is present solely at the a mplifying loci. Mutations in the DNA-binding domains of dE2F or dDP reduce amplification, and in these mutants specific localization of ORC2 to amplif ication loci is lost. Interestingly, a dE2F mutant predicted to lack the ca rboxy-terminal transcriptional activation and RE-binding domain does not ab olish ORC2 localization and shows premature chorion amplification. The effe ct of the mutations in the heterodimer subunits suggests that E2F controls not only the onset of S phase but also origin activity within S phase.