cis-acting sequences responsible for anterior localization of bicoid mRNA in Drosophila embryos

The anterior body pattern of Drosophila melanogaster is specified in large part by the protein product of the bicoid (bcd) gene which functions as a graded morphogen with its peak of expression at the anterior pole of the embryo. Formation of the gradient is dependent on prior localization of bcd messenger RNA at the anterior pole of the egg cell during oogenesis. Here we demonstrate that a discrete portion of the bcd mRNA is necessary for anterior localization of the bcd transcript and is sufficient to cause localization of heterologous transcripts. The sequences responsible for localization appear to span an interval of about 625 base pairs in the 3' untranslated portion of the bcd mRNA and to include regions capable of forming extensive secondary structure. Transcripts from bcd are synthesized predominantly, if not exclusively, in the nurse cells and then transported to the oocyte by connections at the prospective anterior pole. Our findings support the proposal that bcd transcripts are selectively recognized and trapped as they enter the anterior tip of the oocyte, and suggest that this localization process is mediated by anchored sequence-specific receptors in the oocyte cytoplasm.