Requirement for the vasa RNA helicase in gurken mRNA localization

Localization of specific mRNAs to distinct sites within the Drosophila oocyte is an early and key step in establishing the anterior-posterior and dorsal-ventral axes. We describe a new function for the RNA helicase encoded by the "posterior" group gene vasa (vas) in control of localization of the mRNA encoded by the "dorsal-ventral" patterning gene gurken (grk). Two new ethyl methane sulfonate-induced, female sterile alleles of vas have been isolated. In these mutants grk mRNA fails to become localized properly and GRK protein is barely detectable. Surprisingly fs(1)K10, a recessive female sterile mutation that results in mislocalization of GRK mRNA to the anterior end of the oocyte, is epistatic to these vas alleles. This result demonstrates that GRK protein levels sufficient to dorsalize the egg chamber can accumulate in vas mutants, if fs(1)K10 is also mutant. Taken together these results suggest that regulation of GRK mRNA localization normally occurs, directly or indirectly, through the VAS RNA-dependent RNA helicase and may suggest that accumulation of GRK protein normally depends on GRK mRNA localization.