  Zygotic genome activation (ZGA), the onset of transcription after initial
quiescence, is a major developmental step in many species, which occurs after
ten cell divisions in Zebrafish embryos. How transcription factor-chromatin
interactions evolve during early development to support ZGA is largely unknown.
We established single molecule tracking in live developing Zebrafish embryos
using reflected light-sheet microscopy to visualize the general transcription
factor TATA-binding protein (TBP), and developed a novel data acquisition and
analysis scheme to extract kinetic information during fast cell cycles. The
chromatin-bound fraction of TBP increases during early development, compatible
with increasing transcriptional activity. By quantifying TBP and DNA
concentrations and their binding kinetics, we device a physical model of how
the nuclear volume, which decreases during early development, enhances
TBP-chromatin associations. Our single molecule data suggest that the shrinking
nucleus is a major driving force and timer of ZGA in Zebrafish embryos.
