MITOCHONDRIAL-DNA OF THE SEA-ANEMONE, METRIDIUM SENILE (CNIDARIA) - PROKARYOTE-LIKE GENES FOR TRNA(F-MET) AND SMALL-SUBUNIT RIBOSOMAL-RNA, AND STANDARD GENETIC-CODE SPECIFICITIES FOR AGR AND ATA CODONS

The nucleotide sequence of a segment of the mitochondrial DNA (mtDNA) molecule of the sea anemone Metridium senile (phylum Cnidaria, class A nthozoa, order Actiniaria) has been determined, within which have been identified the genes for respiratory chain NADH dehydrogenase subunit 2 (ND2), the small-subunit rRNA (s-rRNA), cytochrome c oxidase subuni t II (COII), ND4, ND6, cytochrome b (Cyt b), tRNA(f-Met) and the large -subunit rRNA (1-rRNA). The eight genes are arranged in the order give n and are all transcribed from the same strand of the molecule. The ov erall order of the M. senile mt-genes differs from that of other metaz oan mtDNAs. In M. senile mt-protein genes, AGA and AGG codons appear t o have the standard genetic code specification of arginine, rather tha n serine as found for other invertebrate mt-genetic codes. Also, ATA h as the standard genetic code specification of isoleucine. TGA occurs i n three M. senile mt-protein genes and may specify tryptophan as in ot her metazoan, protozoan, and some fungal mt-genetic codes. The M. seni le mt-rRNA(f-Met) gene has primary and secondary structure features cl osely resembling those of the Escherichia coli initiator tRNA, includi ng standard dihydrouridine and T psi C loop sequences and a mismatch p air at the top of the aminoacyl stem. Determinations of the 5' and 3' end nucleotides of the M. senile mt-s-rRNAs indicated that these molec ules have a homogenous size of 1,081 ntp, larger than any other known metazoan mt-s-rRNAs. Consistent with its larger size, the M. senile mt -s-rRNA can be folded into a secondary structure that more closely res embles that of the E. coli 16S rRNA than can any other metazoan mt-s-r RNA. These findings concerning M. senile mtDNA indicate that most of t he unusual features regarding metazoan mt-genetic codes, rRNAs, and pr obably tRNAs developed after divergence of the Cnidarian line from the ancestral line common to other metazoa.