The mitochondrial genome of the sea anemone Metridium senile (Cnidaria): introns, a paucity of tRNA genes, and a near-standard genetic code.

The circular, 17,443 nucleotide-pair mitochondrial (mt) DNA molecule of the sea anemone, Metridium senile (class Anthozoa, phylum Cnidaria) is presented. This molecule contains genes for 13 energy pathway proteins and two ribosomal (r) RNAs but, relative to other metazoan mtDNAs, has two unique features: only two transfer RNAs (tRNA(f-Met) and tRNA(Trp)) are encoded, and the cytochrome c oxidase subunit I (COI) and NADH dehydrogenase subunit 5 (ND5) genes each include a group I intron. The COI intron encodes a putative homing endonuclease, and the ND5 intron contains the molecule's ND1 and ND3 genes. Most of the unusual characteristics of other metazoan mtDNAs are not found in M. senile mtDNA: unorthodox translation initiation codons and partial translation termination codons are absent, the use of TGA to specify tryptophan is the 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-1-rRNA have the least deviation from Escherichia coli 16S and 23S rRNAs of all known metazoan mt-rRNAs. These observations indicate that most of the genetic variations previously reported in metazoan mtDNAs developed after Cnidaria diverged from the common ancestral line of all other Metazoa.