Abstract

The newly fertilized embryo is an unusually large and undifferentiated cell. In many species, including Drosophila melanogaster, the first hours of development are spent partitioning that large cell into thousands of smaller cells before zygotic genome activation (ZGA) and cell cycle slowing at a developmental transition known as the Mid-Blastula Transition (MBT). The timing of the MBT is controlled by the increasing Nuclear to Cytoplasmic (N/C) ratio as the embryo divides without growth. We have sought to characterize the molecular mechanisms which allow for N/C ratio sensing in the early embryo. We have found that a large pool of maternally provided histone protein is consumed by the burgeoning amount of zygotic DNA . We show that this leads to de-repression of the cell cycle inhibitor Chk1, which directly interacts with the N-terminal tail of histone H3. This provides a direct link between the N/C ratio and cell cycle slowing at the MBT and a surprising novel role for histones as cell cycle regulators. At the same time, as the maternal supply of H3 is exhausted the amount of its variant, H3.3 increases on the zygotic chromatin. In other tissues H3.3 is known to associate with active promoters and enhancers as well as heterochromatin domains, suggesting a link between H3.3 incorporation and chromatin changes that have long been observed during ZGA . These changes in nuclear H3 and H3.3 nuclear import and incorporation are sensitive to the local N/C ratio. Thus, changes in the nuclear availability of H3 may contribute to cell cycle slowing and transcriptional activation through multiple mechanisms during the MBT . Ongoing work seeks to understand the affects of histone H3 outside of chromatin in later stages of development and to measure more global changes in nuclear composition leading up to the MBT .