EXPRESSION OF A GENE FOR MAJOR MITOCHONDRIAL PROTEIN, ADP ATP TRANSLOCASE, DURING EMBRYOGENESIS IN THE ASCIDIAN HALOCYNTHIA-RORETZI/

The ADP/ATP translocase is the most abundant integral protein of the i nner mitochondrial membrane and it is encoded by the nuclear DNA. Beca use mitochondria in the ascidian egg appear to be segregated into blas tomeres of muscle lineage, we examined the expression of a gene for AD P/ATP translocase during embryogenesis of the ascidian Halocynthia ror etzi. Sequence analysis of a cDNA clone for the ascidian ADP/ATP trans locase indicated that ii contains a single open reading frame that enc odes a polypeptide of 304 amino acids. The polypeptide showed extensiv e similarity to mammalian ADP/ATP translocases, with as much as 74% id entity. The genome of H. roretzi contains a single gene, or two genes al most, for the protein. A large amount of maternal mRNA for ADP/ATP translocase was found in unfertilized eggs and early embryos. The amou nt of this mRNA decreased after the 64-cell stage, and the mRNA became barely detectable in tailbud embryos, although zygotic transcript for the protein was evident in adult tissues. Both in situ hybridization and Northern blot analyses demonstrated that the mRNA is distributed i n the entire cytoplasm of unfertilized eggs. The mRNA is segregated du ring embryogenesis not only into blastomeres of muscle lineage but als o into those of non-muscle lineage.