NUCLEOTIDE-SEQUENCE AND GENE ORGANIZATION OF THE STARFISH ASTERINA-PECTINIFERA MITOCHONDRIAL GENOME

The 16,260-bp mitochondrial DNA (mtDNA) from the starfish Asterina pec tinifera has been sequenced. The genes for 13 proteins, two rRNAs and 22 tRNAs are organized in an extremely economical fashion, similar to those of other animal mtDNAs, with some of the genes overlapping each other. The gene organization is the same as that for another echinoder m, sea urchin, except for the inversion of a 4.6-kb segment that conta ins genes for two proteins, 13 tRNAs and the 16S rRNA. Judging from th e organization of the protein coding genes, mammalian mtDNAs resemble the sea urchin mtDNA more than that of the starfish. The region around the 3' end of the 12S rRNA gene of the starfish shows a high similari ty with those for vertebrates. This region encodes a possible stem and loop structure; similar potential structures occur in this region of vertebrate mtDNAs and also in nonmitochondrial small subunit rRNA. A s imilar stem and loop structure is also found at the 3' end of the 16S rRNA genes in A. pectinifera, in another starfish Pisaster ochraceus, in vertebrates and in Drosophila, but not in sea urchins. The full seq uence data confirm the presumption that AGA/AGG, AUA and AAA codons, r espectively, code for serine, isoleucine, and asparagine in the starfi sh mitochondria, and that AGA/AGG codons are read by tRNA(GCU)(Ser), w hich possesses a truncated dihydrouridine arm, that was previously sug gested from a partial mtDNA sequence. The structural characteristics o f tRNAs and possible mechanisms for the change in the mitochondrial ge netic code are also discussed.