The role of RBF in the introduction of G(1) regulation during Drosophila embryogenesis

The first appearance of G(1) during Drosophila embryogenesis, at cell cycle 17, is accompanied by the downregulation of E2F-dependent transcription. M utant alleles of rbf were generated and analyzed to determine the role of R BF in this process. Embryos lacking both maternal and zygotic RBF products show constitutive expression of PCNA and RNR2, two E2F-regulated genes, ind icating that RBF is required for their transcriptional repression, Despite the ubiquitous expression of E2F target genes, most epidermal cells enter G (1) normally. Rather than pausing in G(1) until the appropriate time For ce ll cycle progression, many of these cells enter an ectopic S-phase. These r esults indicate that the repression of E2F target genes by RBF is necessary for the maintenance but not the initiation of a G(1) phase. The phenotype of RBF-deficient embryos suggests that rbf has a function that is complemen tary to the roles of dacapo and fizzy-related in the introduction of G(1) d uring Drosophila embryogenesis.