RT Journal Article SR Electronic T1 Synthetic deconstruction of hunchback regulation by Bicoid JF bioRxiv FD Cold Spring Harbor Laboratory SP 2021.09.06.459125 DO 10.1101/2021.09.06.459125 A1 Goncalo Fernandes A1 Huy Tran A1 Maxime Andrieu A1 Youssoupha Diaw A1 Carmina Perez-Romero A1 Cécile Fradin A1 Mathieu Coppey A1 Aleksandra M. Walczak A1 Nathalie Dostatni YR 2021 UL http://biorxiv.org/content/early/2021/09/07/2021.09.06.459125.1.abstract AB During development, cell identity is established reproducibly among individuals through the expression of specific genes at the correct time and correct location in space. How genes extract and combine both positional and temporal information from different transcription factor (TF) profiles along polarity axes remain largely unexplored. Here, we showcase the classic hunchback gene in fruit fly embryos, with focus on 3 of its main TFs: Bicoid, Zelda and Hunchback proteins. We constructed a series of synthetic MS2 reporters, where the numbers and combination of binding sites for each TF are varied. Using live imaging of transcription dynamics by these synthetic reporters and modeling tools, we show that i) a Bicoid-only synthetic reporter needs 3 more Bicoid binding sites than found in the hunchback promoter to recapitulates almost all spatial features of early hb expression but takes more time to reach steady state; ii) Hunchback and Zelda binding sites combined with Bicoid sites both reduce the time to reach steady state and increase expression at a different step in the activation process: Zld sites lower the Bicoid threshold required for activation while Hb sites increase the polymerase firing rate and reduce bursting; iii) the shift of the Bicoid-only reporter induced by a reduction by half of Bicoid concentrations indicates that the decay length of the Bicoid activity gradient is lower than the decay length of the Bicoid protein gradient. Altogether, this work indicates that Bicoid is the main source of positional information for hunchback expression and places back the Bicoid system within the physical limits of an equilibrium model.Competing Interest StatementThe authors have declared no competing interest.