Dynamics of hunchback translation in real time and at single mRNA resolution in the Drosophila embryo

The Hunchback (Hb) transcription factor is critical for anterior-posterior patterning of the Drosophila embryo. Despite the maternal hb mRNA acting as a paradigm for translational regulation, due to its repression in the posterior of the embryo, little is known about the translatability of zygotically transcribed hb mRNAs. Here we adapt the SunTag system, developed for imaging translation at single mRNA resolution in tissue culture cells, to the Drosophila embryo to study the translation dynamics of zygotic hb mRNAs. Using singlemolecule imaging in fixed and live embryos, we provide evidence for translational repression of zygotic SunTag-hb mRNAs. While the proportion of SunTag-hb mRNAs translated is initially uniform, translation declines from the anterior over time until it becomes restricted to a posterior band in the expression domain. We discuss how regulated hb mRNA translation may help establish the sharp Hb expression boundary, which is a model for precision and noise during developmental patterning. Overall, our data show how use of the SunTag method on fixed and live embryos is a powerful combination for elucidating spatiotemporal regulation of mRNA translation in Drosophila.