Mechanism and Regulation of Cdc25/Twine Protein Destruction in Embryonic Cell-Cycle Remodeling

Background: In Drosophila embryos, the midblastula transition (MBT) dramatically remodels the cell cycle during the 14th interphase. Before the MBT, each cycle is composed of only a short S phase and mitosis. At the MBT, S phase is dramatically lengthened by the onset of late replication, and a G2 phase is introduced. Both changes set the stage for gastrulation and require downregulation of Cdc25 phosphatase, which was previously attributed to the elimination of its transcripts at the MBT. Results: Premature removal of cdc25 transcripts by RNAi did not affect progression to the MBT. Instead, an antibody against the Cdc25 isoform Twine showed that Twine protein was abundant and stable until the MBT, when it was destabilized and rapidly eliminated. Persistence of pre-MBT levels of Twine was sufficient to prevent cell-cycle slowing. Twine protein destruction was timed by the nucleocytoplasmic ratio and depended on the activation of zygotic transcription at the MBT, including expression of the gene tribbles, whose activity was sufficient to trigger Twine destruction and was required for prompt Twine disappearance. Conclusions: We propose that the developmentally regulated destruction of Twine protein is a critical switch that contributes to the cell-cycle change at the MBT, including the addition of a G2 phase and onset of late replication. Moreover, we show that this destruction is triggered by the nucleocytoplasmic ratio-dependent onset of zygotic transcription of tribbles and other unknown genes.