Mutations of the Drosophila dDP,dE2F, and cyclin E Genes Reveal Distinct Roles for the E2F-DP Transcription Factor and Cyclin E during the G₁-S Transition

ABSTRACT

Activation of heterodimeric E2F-DP transcription factors can drive the G₁-S transition. Mutation of the Drosophila melanogaster dE2F gene eliminates transcriptional activation of several replication factors at the G₁-S transition and compromises DNA replication. Here we describe a mutation in the Drosophila dDP gene. As expected for a defect in the dE2F partner, this mutation blocks G₁-S transcription ofDmRNR2 and cyclin E as previously described for mutations of dE2F. Mutation of dDP also causes an incomplete block of DNA replication. When S phase is compromised by reducing the activity of dE2F-dDP by either a dE2F ordDP mutation, the first phenotype detected is a reduction in the intensity of BrdU incorporation and a prolongation of the labeling. Notably, in many cells, there was no detected delay in entry into this compromised S phase. In contrast, when cyclin E function was reduced by a hypomorphic allele combination, BrdU incorporation was robust but the timing of S-phase entry was delayed. We suggest that dE2F-dDP contributes to the expression of two classes of gene products: replication factors, whose abundance has a graded effect on replication, and cyclin E, which triggers an all-or-nothing transition from G₁ to S phase.

ACKNOWLEDGMENTS

We thank Nick Dyson for dDP cDNAs and anti-dDP monoclonal antibodies, Ting Wu for mutant stocks from the 49F region, Carole Seum and Pierre Spierer for the dE2F¹⁶⁴ mutant stock, Shelagh Campbell and Tin Tin Su for library cDNA and primers, Peter Follette for dMCM3 cDNA, Terry Orr-Weaver for communicating results prior to publication, and Mark Peifer and Jenny Adam for comments on the manuscript.

This work was initiated while R.J.D. was supported by a fellowship from the Cancer Research Fund of the Damon Runyon-Walter Winchell Foundation (DRG-1161). This work was funded in part by a grant from the National Institutes of Health to P.H.O. (GM37193).