THE MITOCHONDRIAL GENOME OF THE SEA-ANEMONE METRIDIUM SENILE (CNIDARIA) - INTRONS, A PAUCITY OF TRANSFER-RNA GENES, AND A NEAR-STANDARD GENETIC-CODE

The circular, 17,443 nucleotide-pair mitochondrial (mt) DNA molecule o f the sea anemone, Metridium senile (class Anthozoa, phylum Cnidaria) is presented. This molecule contains genes for 13 energy pathway prote ins and two ribosomal (r) RNAs but, relative to other metazoan mtDNAs, has two unique features: only two transfer RNAs (tRNA(t-Met) and tRNA (Trp)) are encoded, and the cytochrome c oxidase subunit I (COI) and N ADH dehydrogenase subunit 5 (ND5) genes each include a group I intron. The COI intron encodes a putative homing endonuclease, and the ND5 in tron contains the molecule's ND1 and ND3 genes. Most of the unusual ch aracteristics of other metazoan mtDNAs are not found in M. senile mtDN A: unorthodox translation initiation codons and partial translation te rmination codons are absent, the use of TGA to specify tryptophan is t he only genetic code modification, and both encoded tRNAs have primary and secondary structures closely resembling those of standard tRNAs. Also, with regard to size and secondary structure potential, the mt-s- rRNA and mt-l-rRNA have the least deviation from Escherichia coli 16S and 23S rRNAs of all known metazoan mt-rRNAs. These observations indic ate that most of the genetic variations previously reported in metazoa n mtDNAs developed after Cnidaria diverged from the common ancestral l ine of all other Metazoa.