Presence of mitochondria-type ribosomes outside mitochondria in germ plasmof Drosophila embryos

Mitochondrially encoded large and small ribosomal RNAs (mtlrRNA and mtsrRNA ) are transported out of mitochondria to polar granules, the distinctive or ganelles of germ plasm in Drosophila. Reduction of the extramitochondrial m tlrRNA amount leads to the failure of embryos to form the germ-line progeni tors, or pole cells, suggesting that mtlrRNA, along with mtsrRNA, functions on the polar granules to specify the germ line. In this study, we provide several lines of evidence showing that there are mitochondria-type ribosome s on the polar granules during a short period before pole cell formation. O ur ultrastructural analysis reveals that these ribosomes include both mitoc hondria) rRNAs and at least two mitochondria) ribosomal proteins (S12 and L 7/L12). Furthermore, these ribosomes are integrated into well developed pol ysomes on the surface of polar granules. We propose that translation depend ent on mitochondria-type ribosomes is an important mechanism underlying ger m-line formation.