Live Imaging of Bicoid-Depe

The earlyDrosophila embryo is an idealmodel to understand the transcriptional regulation of well-defined patterns of gene expression in adevelopingorganism [1]. In this system, snapshots of transcription measurements obtained by RNA FISH on fixed samples [2, 3] cannot provide the temporal resolution needed to distinguish spatial heterogeneity [3] from inherent noise [4, 5]. Here, we used the MS2-MCP system [6, 7] to visualize in living embryos nascent transcripts expressed from the canonical hunchback (hb) promoter [8, 9] under the control of Bicoid (Bcd) [10]. The hb-MS2 reporter is expressed as synchronously as endogenous hb in the anterior half of the embryo, but unlike hb it is also active in the posterior, though more heterogeneously and more transiently than in the anterior. The length and intensity of active transcription periods in the anterior are strongly reduced in absence of Bcd, whereas posterior ones are mostly Bcd independent. This posterior noisy signal decreases progressively through nuclear divisions, so that the MS2 reporter expression mimics the known anterior hb pattern at cellularization. We propose that the establishment of the hb pattern relies on Bcd-dependent lengthening of transcriptional activity periods in the anterior and may require two distinct repressionmechanisms in the posterior.