Expression of a Gene for Major Mitochondrial Protein, ADP/ATP Translocase, during Embryogenesis in the Ascidian Halocynthia roretzi

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Abstract
The ADP/ATP translocase is the most abundant integral protein of the inner mitochondrial membrane and it is encoded by the nuclear DNA. Because mitochondria in the ascidian egg appear to be segregated into blastomeres of muscle lineage, we examined the expression of a gene for ADP/ATP translocase during embryogenesis of the ascidian Halocynthia roretzi. Sequence analysis of a cDNA clone for the ascidian ADP/ATP translocase indicated that it contains a single open reading frame that encodes a polypeptide of 304 amino acids. The polypeptide showed extensive similarity to mammalian ADP/ATP translocases, with as much as 74% identity. The genome of H. roretzi contains a single gene, or two genes at most, for the protein. A large amount of maternal mRNA for ADP/ATP translocase was found in unfertilized eggs and early embryos. The amount 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 in the entire cytoplasm of unfertilized eggs. The mRNA is segregated during embryogenesis not only into blastomeres of muscle lineage but also into those of non‐muscle lineage.