ABSTRACT
Following completion of meiosis, DNA replication must be repressed until fertilization. In Drosophila, this replication block requires the products of the pan gu (png), plutonium (plu) and giant nuclei (gnu) genes. These genes also ensure that S phase oscillates with mitosis in the early division cycles of the embryo. We have identified the png gene and shown that it encodes a Ser/Thr protein kinase expressed only in ovaries and early embryos, and that the predicted extent of kinase activity in png mutants inversely correlates with the severity of the mutant phenotypes. The PLU and PNG proteins form a complex that has PNG-dependent kinase activity, and this activity is necessary for normal levels of mitotic cyclins. Our results reveal a novel protein kinase complex that controls S phase at the onset of development apparently by stabilizing mitotic cyclins.
Subject(s)
Drosophila Proteins , Drosophila/cytology , Drosophila/enzymology , Mitosis/physiology , Protein Serine-Threonine Kinases/physiology , S Phase/physiology , Amino Acid Sequence , Animals , Animals, Genetically Modified , Cyclins/metabolism , Drosophila/genetics , Drosophila/growth & development , Female , Genes, Insect , Male , Molecular Sequence Data , Mutation , Phylogeny , Protein Serine-Threonine Kinases/chemistry , Protein Serine-Threonine Kinases/genetics , Protein Structure, Tertiary , Restriction MappingABSTRACT
Unfertilized eggs and fertilized embryos from Drosophila mothers mutant for the plutonium (plu) gene contain giant polyploid nuclei resulting from unregulated S-phase. The PLU protein, a 19-kDa ankyrin repeat protein, is present in oocytes and early embryos but is not detectable after the completion of the initial rapid S-M cycles of the embryo. The persistence of the protein during the early embryonic divisions is consistent with a direct role in linking S-phase and M-phase. When ectopically expressed in the eye disc, PLU did not perturb the cell cycle, suggesting that PLU regulates S-phase only in early embryonic development. The pan gu (png) and giant nuclei (gnu) genes also affect the S-phase in the unfertilized egg and early embryo. We show that functional png is needed for the presence of PLU protein. By analyzing png mutations of differing severity, we find that the extent of the png mutant phenotype inversely reflects the level of PLU protein. Our data suggest that PLU protein is required at the time of egg activation and the completion of meiosis.
