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Center for Nonlinear Studies |
Since Turing suggested that cell differentiation in living organisms is mediated by the concentrations of specific chemical factors, called morphogens, much progress has been made in advancing a quantitative understanding of this phenomenon. The first such factor to be identified was the protein Bicoid, which specifies the anterior-posterior axis of the Drosophila melanogaster embryo via a concentration gradient. However, the mechanism of gradient formation remains unknown. One model postulates that gradient shape is determined by an equilibration between diffusion and degradation while protein is synthesized from an anterior-localized source. We use a fusion of Bicoid and the photoswitchable fluorescent protein Dronpa to measure the in vivo rate of Bicoid degradation by a new method we call an optical pulse chase experiment. According to this method, Bicoid has a lifetime of 50 minutes prior to mitotic cycle 14, which declines to 15 minutes during cellularization. Using quantitative western blotting, we show that the total amount of Bicoid in the embryo increases linearly before the onset of zygotic transcription, reaching a maximum quantity of 109 Bicoid molecules in cycle 14. While Bicoid synthesis is not constant, the shape of the gradient is well explained by the measured Bicoid degradation rate and previously published values of the Bicoid diffusion constant. Moreover, we show that optical modulation of the apparent lifetime of Bicoid changes the characteristic length of the gradient according to the prediction of the diffusion/degradation equilibrium model.
Host: Cynthia Reichhardt, cjrx@lanl.gov, 5-1134