GSE41686
A global change in RNA Polymerase II pausing during the Drosophila midblastula transition [MNase-seq]
During animal development, a fertilized egg is initially under the control of maternal products and only starts zygotic transcription after several cell divisions. In animals such as Xenopus, zebrafish and Drosophila, a massive increase in zygotic transcription occurs during the mid-blastula transition (MBT), when cells shift from rapid, synchronous cell divisions without gap phases to prolonged asynchronous divisions. Before MBT, only a few so-called pre-MBT genes are expressed. How transcription is set up during these early stages is poorly understood. For example, paused RNA Polymerase (Pol II) is frequently found at developmental control genes in mammalian embryonic stem cells and Drosophila embryos but when Pol II pausing is first established in the embryo is unknown. We have analyzed the genome-wide Pol II occupancy during the maternal-to-zygotic transition in hand-staged Drosophila embryos. The results show that massive Pol II recruitment and pausing is established during MBT. The ~100 genes that are transcribed before MBT are particularly short, consistent with a need for rapid transcription during these early cell divisions. Remarkably, most of these genes are transcribed without Pol II pausing and this correlates with a TATA-enriched promoter type. This suggests that distinct strategies are used for activation in the early Drosophila embryo and this may reflect general dynamic properties of promoters used throughout development. Overall design: Mnase-seq in staged Drosophila embryos