Dynamics of Free and Chromatin-Bound Histone H3 during Early Embryogenesis.

During zygotic genome activation (ZGA), the chromatin environment undergoes profound changes, including the formation of topologically associated domains, refinements in nucleosome positioning on promoters, and the emergence of heterochromatin [1-4]. In many organisms, including Drosophila, ZGA is associated with the end of a period of extremely rapid, exponential cleavage divisions that are facilitated by large maternally provided pools of nuclear components. It is therefore imperative that we understand how the supply of chromatin components relative to the exponentially increasing demand affects nuclear and chromatin composition during early embryogenesis. Here, we examine the nuclear trafficking and chromatin dynamics of histones during the cleavage divisions in Drosophila using a photo-switchable H3-Dendra2 reporter. We observe that total H3-Dendra2 in the nucleus decreases with each cleavage cycle. This change in nuclear composition is due to depletion of large pools (>50%) of free protein that are present in the early cycles. We find that the per nucleus import rate halves with each cycle and construct a mathematical model in which increasing histone demand determines the dynamics of nuclear H3 supply. Finally, we show that these changes in H3 availability correspond to a large (∼40%) reduction in global H3 occupancy on the chromatin, which is compensated by the increased incorporation of H3.3. The observed changes in free nuclear H3 and chromatin composition may contribute to the cell-cycle slowing, changes in chromatin structure, and the onset of transcription associated with this developmental stage.