Restoration of pole-cell-forming ability to u.v.-irradiated Drosophila embryos by injection of mitochondrial IrRNA

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Abstract
Screening a cDNA library generated from poly(A)+RNA of Drosophila cleavage embryos, we selected a cDNA clone (pDE20.6). The cDNA hybridized specifically with a poly(A)+RNA that is capable of restoring embryos from u.v.-caused inability of pole cell formation. The RNA hybrid-selected by pDE20.6 was also able to induce pole cells in the anterior region of embryos, if it was coinjected with u.v.-irradiated polar plasm, although the RNA or irradiated polar plasm alone was not effective. Pole cells thus formed in the anterior or in the u.v.-irradiated posterior region were identified by polar granules and nuclear bodies, morphological markers for normal pole cells. Furthermore, the RNA-induced pole cells were able to migrate into gonadal rudiments. The nucleotide sequence of pDE20.6 cDNA insert was highly homologous with the mitochondrial large rRNA (IrRNA) gene, but not with any nuclear DNA sequences. Using pDE20.6 as a primer, a full-length cDNA of mitochondrial IrRNA was generated and cloned. The RNA transcribed in vitro from the cDNA was able to restore pole cell formation. The cDNA hybridized only with a 1.5 kb poly(A)+RNA on a Northern blot. The 1.5 kb RNA sedimented more with the post-mitochondrial (P3) fraction than with the mitochondrial (P2) fraction, while the majority of transcripts from another mitochondrial gene was detected in the P2 fraction.