Translational repression by Bicoid: competition for the cap

Long-range formation of the Bicoid gradient requires multiple dynamic modes that spatially vary across the embryo

Morphogen gradients provide essential positional information to gene networks through their spatially heterogeneous distribution, yet how they form is still hotly contested, with multiple models proposed for different systems. Here, we focus on the transcription factor Bicoid (Bcd), a morphogen that forms an exponential gradient across the anterior-posterior (AP) axis of the early Drosophila embryo. Using fluorescence correlation spectroscopy we find there are spatial differences in Bcd diffusivity along the AP axis, with Bcd diffusing more rapidly in the posterior. We establish that such spatially varying differences in Bcd dynamics are sufficient to explain how Bcd can have a steep exponential gradient in the anterior half of the embryo and yet still have an observable fraction of Bcd near the posterior pole. In the nucleus, we demonstrate that Bcd dynamics are impacted by binding to DNA. Addition of the Bcd homeodomain to eGFP::NLS qualitatively replicates the Bcd concentration profile, suggesting this domain regulates Bcd dynamics. Our results reveal how a long-range gradient can form while retaining a steep profile through much of its range.

Regulating translation of maternal messages: multiple repression mechanisms

The dowry of mRNAs and proteins that mothers provide their progeny as part of a common developmental strategy to permit rapid embryogenesis necessitates precise translational regulation of the deposited mRNAs. Recent studies with Drosophila uncovered diverse mechanisms to control translation of the transcripts for genes that control the cell cycle and embryonic patterning. The newly delineated mechanisms include: alternative ways to disrupt eIF4E action and the formation of the preinitiation complex b y the eIF4E homologous protein, d4EHP; recruitment of the deadenylase complex by the SMAUG and PUMILIO proteins; both poly(A)-dependent and -independent promotion of translation by the PNG kinase complex; and 5' cap-independent translational regulation b y BRUNO.