The contribution of E2F-regulated transcription to Drosophila PCNA gene function.

E2F proteins control cell cycle progression by predominantly acting as either activators or repressors of transcription. How the antagonizing activities of different E2Fs are integrated by cis-acting control regions into a final transcriptional output in an intact animal is not well understood. E2F function is required for normal development in many species, but it is not completely clear for which genes E2F-regulated transcription provides an essential biological function. To address these questions, we have characterized the control region of the Drosophila PCNA gene. A single E2F binding site within a 100-bp enhancer is necessary and sufficient to direct the correct spatiotemporal program of G1-S-regulated PCNA expression during development. This dynamic program requires both E2F-mediated transcriptional activation and repression, which, in Drosophila, are thought to be carried out by two distinct E2F proteins. Our data suggest that functional antagonism between these different E2F proteins can occur in vivo by competition for the same binding site. An engineered PCNA gene with mutated E2F binding sites supports a low level of expression that can partially rescue the lethality of PCNA null mutants. Thus, E2F regulation of PCNA is dispensable for viability, but is nonetheless important for normal Drosophila development.