REGULATION OF CELL CYCLE ARREST BY dE2F2 DURING OOGENESIS. P.M. Cayirlioglu 1, P. Bonnette 3, M.M. Dickson 1, R.J. Duronio 1,2,3,4. 1) Dept Biology; 2) Lineberger Comprehensive Cancer Center; 3) Program in Molecular Biology and Biotechnology; 4) Curriculum in Genetics and Molecular Biology, Univ of North Carolina, Chapel Hill, NC.
The E2F family of transcription factors imparts both positive and negative influences on the cell cycle. In the positive role, E2F turns on genes required for cell cycle progression and DNA synthesis. In the negative role, E2F acts as a transcriptional repressor by binding a pRb family member . However, the relative contribution of the repressor and activator roles of E2F to the control of cell division is unclear. E2Fs are heterodimers composed of E2F and DP subunits. Whereas there are 6 E2F and 2 DP genes in mammals, Drosophila contains only two E2F (dE2F, dE2F2) and a single DP gene (dDP). Both dE2F and dDP are essential and function as positive regulators of the cell cycle. However, the role of dE2F2 in cell cycle progression and development is unknown. We generated mutations of dE2F2 to investigate its cell cycle regulatory role. Mutants homozygous for a dE2F2 null or hypomorphic allele are viable, but female sterile. Eggs laid by these mutant females have thin eggshells. At stage 10 of oogenesis, the follicle cells around the oocyte exit from endoreduplication cycles and begin to amplify two chorion gene clusters which can be visualized by BrdU labeling or by immuno-detection of ORCs. In dE2F2 mutant ovaries the follicle cells are unable to exit the endoreduplication cycles at stage 10 and continue replicating the whole genome, resulting in an inappropriate increase in ploidy. Consistent with continued genomic replication, Orc2, Orc5 and Cdc45 proteins are mislocalized throughout follicle cell nucleus in dE2F2 mutant egg chambers at stage 10. All these phenotypes are also observed with a dE2F2 allele that cannot bind pRb and can be rescued with a wild type dE2F2 transgene. These results indicate that dE2F2 functions as a negative regulator of cell cycle progression, possibly by acting in a pRb containing repressor complex.