PROTEIN ENCODED BY THE EXUPERANTIA GENE IS CONCENTRATED AT SITES OF BICOID MESSENGER-RNA ACCUMULATION IN DROSOPHILA NURSE CELLS BUT NOT IN OOCYTES OR EMBRYOS

Formation of the anteroposterior gradient of the bicoid (bcd) protein morphogen in Drosophila occurs as a consequence of the prelocalization of bcd mRNA to the anterior pole of the oocyte. bcd mRNA localization requires at least three genes, with the exuperantia (exu) gene acting earliest in the pathway. We have cloned and characterized the exu gene. Notably, we find that exu protein is colocalized with bcd mRNA during the early phase of localization, when bcd mRNA is positioned at the apical regions of the nurse cells. However, there is no colocalization at the later stage when bcd mRNA is transferred from the nurse cells to the anterior pole of the oocyte. There is no detectable exu protein in embryos. Therefore, exu acts only in initiating bcd mRNA localization and does not play a persistent role in that process.