Transcriptional. repressor functions of Drosophila E2F1 and E2F2 cooperate to inhibit genomic DNA synthesis in ovarian follicle cells

Individual members of the E2F/DP protein family control cell cycle progression by acting predominantly as an activator or repressor of transcription. In Drosophila melanogaster the E2f1, E2f2, Bp, and Rbf1 genes all contribute to replication control in ovarian follicle cells, which become 16C polyploid and subsequently undergo chorion gene amplification late in oogenesis. Mutation of E2f2, Dp, or Rbf1 causes ectopic DNA replication throughout the follicle cell genome during gene amplification cycles. Here we show by both reverse transcription-PCR and DNA microarray analysis that the transcripts of prereplication complex (pre-RC) genes are elevated compared to the wild type in E2f2, Dp, and Rbf1 mutant follicle cells. For some genes the magnitude of this transcriptional derepression is greater in Rbf1 than in E2f2 mutants. These differences correlate with differences in the magnitude of the replication defects in follicle cells, which attain an inappropriate 32C DNA content in both Rbf1 and Dp mutants but not in E2f2 mutants. The ectopic genomic replication of E2f2 mutant follicle cells can be suppressed by reducing the Orc2, Orc5, or Mcm2 gene dose by half, indicating that small changes in pre-RC gene expression can affect DNA synthesis in these cells. We conclude that RBF1 forms complexes with both E2F1/DP and E2F2/DP that cooperate to repress the expression of pre-RC genes, which helps confine DNA synthesis to sites of gene amplification. In contrast, E2F1 and E2F2 repressors function redundantly for some genes in the embryo. Thus, the relative functional contributions of E2F1 and E2F2 to gene expression and cell cycle control depends on the developmental context.