Maternally synthesized localized determinants play an important role in cell fate specification in early Drosophila embryos. Since only a third of the genes in Drosophila have been identified genetically, we carried out a systematic screen for polar localized maternal RNAs in the early embryo as a means of identifying novel molecules that might serve important developmental functions. Anterior-and posterior-specific directional cDNA libraries were constructed using RNA purified from anterior-or posterior-poles cut off early embryos. These libraries were used in a differential screen for cDNAs representing polar-localized maternal RNAs. Five such clones were identified, encoding cyclin B RNA, Hsp83 RNA, 28S rRNA, mitochondrial COl RNA and mitochondria/16S lrRNA (16S RNA). Maternal Hsp83 transcripts are localized to the posterior pole of the early embryo by a novel mechanism involving generalized RNA degradation and local protection at the posterior. This protection is dependent on the integrity of the polar plasm, suggesting that Hsp83 RNA is a component of the polar plasm. Results from antisense oligodeoxynucleotide injection experiments suggest that Hsp83 is required for the formation/maintenance of germ cells. The 16S RNA is highly concentrated at the posterior pole of newly fertilized Drosophila eggs, a process dependent on the integrity of the polar plasm. The localization pattern of 16S RNA does not correlate well with either the distribution or the activity of mitochondria in early embryos. Taken together with previously published data, these results suggest that the 16S RNA is exported from the mitochondria into the posterior polar plasm and that it functions in pole cell formation. In addition to the localized RNAs identified in the differential screen, a novel anteriorly localized RNA was identified. This mRNA encodes a Drosophila homolog of mammalian adducin, a membrane-cytoskeletal protein that functions in the assembly of the spectrin-actin network. A comparison of the spatial distribution of bicoid and Adducin-like transcripts in maternal effect RNA-localization mutants indicates that different genetic pathways exist for localization of mRNAs to the anterior pole of the oocyte and early embryo.
