The temporal and spatial distribution pattern of maternal exuperantia protein: evidence for a role in establishment but not maintenance of bicoid mRNA localization.

The exuperantia (exu) gene of Drosophila melanogaster plays a fundamental role in the establishment of polarity of the oocyte and early embryo by ensuring the proper localization of the mRNA of the bicoid (bcd) gene to anterior regions of the oocyte. We have isolated and sequenced the exu gene, sequenced its female-specific transcript and a mutant allele of exu that affects primarily exu's female germline function, and determined the temporal and spatial pattern of exu protein expression during oogenesis. The exu protein is basic, with at least one basic residue being identified as necessary for exu function in the female germline, and is present transiently during oogenesis. Our results suggest that exu is not required for the maintenance of bcd mRNA localization during late stages of oogenesis and early embryogenesis, but rather for the establishment of bcd mRNA localization in the developing oocyte. We propose that the exu protein may serve to modify a component that binds bcd mRNA or to modify the bcd message itself, or may perform a role in docking the bcd mRNA at its site of localization in the developing oocyte.