Mitochondrial small ribosomal RNA is present on polar granules in early cleavage embryos of Drosophila melanogaster

In Drosophila, formation of the germline progenitors, the pole cells, is in duced by polar plasm localized in the posterior pole region of early embryo s. The polar plasm contains polar granules, which act as a repository for t he factors required for pole cell formation. It has been postulated that th e factors are stored as mRNA and are later translated on polysomes attached to the surface of polar granules. Here, the identification of mitochondria l small ribosomal RNA (mtsrRNA) as a new component of polar granules is des cribed. The mtsrRNA was enriched in the polar plasm of the embryos immediat ely after oviposition and remained in the polar plasm throughout the cleava ge stage until pole cell formation. In situ hybridization at an ultrastruct ural level revealed that mtsrRNA was enriched on the surface of polar granu les in cleavage embryos. Furthermore, the localization of mtsrRNA in the po lar plasm depended on the normal function of oskar, vasa and tudor genes, w hich are all required for pole cell formation. The temporal and spatial dis tribution of mtsrRNA is essentially identical to that of mitochondrial larg e ribosomal RNA (mtlrRNA), which has been shown to be required for pole cel l formation. Taken together, it is speculated that mtsrRNA and mtlrRNA are part of the translation machinery localized to polar granules, which is ess ential for pole cell formation.