Biochemical Fractionation of Time-Resolved Drosophila Embryos Reveals Similar Transcriptomic Alterations in Replication Checkpoint and Histone mRNA Processing Mutants.

In higher eukaryotes, maternally provided gene products drive the initial stages of embryogenesis until the zygotic transcriptional program takes over, a developmental process called the midblastula transition (MBT). In addition to zygotic genome activation, the MBT involves alterations in cell-cycle length and the implementation of DNA damage/replication checkpoints that serve to monitor genome integrity. Previous work has shown that mutations affecting histone mRNA metabolism or DNA replication checkpoint factors severely impact developmental progression through the MBT, prompting us to characterize and contrast the transcriptomic impact of these genetic perturbations. In this study, we define gene expression profiles that mark early embryogenesis in Drosophila through transcriptomic analyses of developmentally staged (early syncytial versus late blastoderm) and biochemically fractionated (nuclear versus cytoplasmic) wild-type (wt) embryos. We then compare the transcriptomic profiles of loss-of-function mutants of the Chk1/Grapes replication checkpoint kinase and the stem loop binding protein (SLBP), a key regulator of replication-dependent histone mRNAs. Our analysis of RNA spatial and temporal distribution during embryogenesis offers new insights into the dynamics of early embryogenesis. In addition, we find that grp and Slbp mutant embryos display profound and highly similar defects in gene expression, most strikingly in zygotic gene expression, compromising the transition from a maternal to a zygotic regulation of development.