Distinct cis-acting elements mediate targeting and clustering of Drosophila polar granule mRNAs

Specification and development of Drosophila germ cells depend on molecular determinants within the germ plasm, a specialized cytoplasmic domain at the posterior of the embryo. Localization of numerous mRNAs to the germ plasm occurs by their incorporation, as single-transcript ribonucleoprotein (RNP) particles, into complex RNP granules called polar granules. Incorporation of mRNAs into polar granules is followed by recruitment of additional like transcripts to form discrete homotypic clusters. The cis-acting localization signals that target mRNAs to polar granules and promote homotypic clustering remain largely uncharacterized. Here, we show that the polar granule component (pgc) and germ cell-less (gcl) 3' untranslated regions contain complex localization signals comprising multiple, independently weak and partially functionally redundant localization elements (LEs). We demonstrate that targeting of pgc to polar granules and self-assembly into homotypic clusters are functionally separable processes mediated by distinct classes of LEs. We identify a sequence motif shared by other polar granule mRNAs that contributes to homotypic clustering. Our results suggest that mRNA localization signal complexity may be a feature required by the targeting and self-recruitment mechanism that drives germ plasm mRNA localization.