E2F, a heterodimer consisting of E2F and DP, is a sequence-specific transcription factor that regulates genes required for DNA synthesis, and cell proliferation. Interaction of the tumor suppressor protein Rb with E2F causes the complex to act as a transcriptional repressor. While the functions of Rb and E2F have been heavily studied in cell culture systems, studies in vivo have been complicated by the multiplicity of Rb and E2F family members. C. elegans is an excellent model system to address Rb/E2F function because it contains a single Rb (
lin-35), a single DP (
dpl-1), and only three E2Fs, including
efl-1. Currently,
efl-1 is the only E2F for which an in vivo phenotype has been identified. Our previous microarray analysis indicated that all three of these proteins have enriched expression in the germline, so we have begun to investigate the roles of
lin-35,
dpl-1 and
efl-1 in germline development and embryogenesis. While
lin-35 mutants appear relatively normal, both
dpl-1 and
efl-1 mutants display embryonic lethality at or just after the one-cell stage. To investigate the molecular basis of these phenotypes, we have performed microarray analysis comparing
lin-35,
dpl-1 or
efl-1 mutant dissected gonads to wild type gonads. We found that
dpl-1 and
efl-1 regulate essentially the same set of genes, while the set of
lin-35-regulated genes is distinct. This observation is consistent with the observed phenotypes, and suggests that DPL-1 and EFL-1 function together as a heterodimer independently of
lin-35 Rb in the germline. We are focusing on the set of 91 genes that are transcriptionally responsive to the both DPL-1 and EFL-1. Of these, 83 out of 91 have higher expression in wild type than in
dpl-1 or
efl-1 mutants, indicating that DPL-1/EFL-1 functions as a transcriptional activator rather than a repressor. The putative promoter regions of these genes have a canonical mammalian E2F consensus binding site. In addition, in situ hybridization analysis of the genes (Kohara database) shows that most have an onset of expression concomitant with the localized expression of EFL-1. We have found that RNAi of 29 target genes produces an embryonic lethal phenotype by affecting various processes, including oocyte maturation, eggshell formation, embryonic polarity, and the cell cycle. Together our results argue that, in the germline, DPL-1/EFL-1 acts as a transcriptional activator, independent of LIN-35 Rb, to activate a genetic program that mediates late events in oogenesis and early events in embryogenesis. Intriguingly, the relationship between DPL-1, EFL-1 and LIN-35 is clearly different in germline development compared to vulval development, suggesting that the Rb/E2F complex has tissue-specific functions.