DEVELOPMENTAL CONTROL OF A G(1)-S TRANSCRIPTIONAL PROGRAM IN DROSOPHILA

We have defined a coordinate program of transcription of S-phase genes (DNA polymerase alpha, PCNA and the two ribonucleotide reductase subu nits) that can be induced by the G(1) cyclin, cyclin E. In Drosophila embryos, this program drives an intricate spatial and temporal pattern of gene expression that perfectly parallels the embryonic program of S-phase control. This dynamic pattern of expression is not disrupted b y a mutation, string, that blocks the cell cycle. Thus, the transcript ional program is not a secondary consequence of cell cycle progression . We suggest that developmental signals control this transcriptional p rogram and that its activation either directly or indirectly drives tr ansition from G(1) to S phase in the stereotyped embryonic pattern.