Platyhelminth mitochondrial DNA: Evidence for early evolutionary origin of a tRNAserAGN that contains a dihydrouridine arm replacement loop, and of serine-specifying AGA and AGG codons

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Abstract
The nucleotide sequence of a segment of the mitochondrial DNA (mtDNA) molecule of the liver flukeFasciola hepatica (phylum Platyhelminthes, class Trematoda) has been determined, within which have been identified the genes for tRNAala, tRNAasp, respiratory chain NADH dehydrogenase subunit I (ND1), tRNAasn, tRNApro, tRNAile, tRNAlys, ND3, tRNAserAGN, tRNAtrp, and cytochromec oxidase subunit I (COI). The 11 genes are arranged in the order given and are all transcribed from the same strand of the molecule. The overall order of theF. hepatica mitochondrial genes differs from what is found in other metazoan mtDNAs. All of the sequenced tRNA genes except the one for tRNAserAGN can be folded into a secondary structure with four arms resembling most other metazoan mitochondrial tRNAs, rather than the tRNAs that contain a TψC arm replacement loop, found in nematode mtDNAs. TheF. hepatica mitochondrial tRNAserAGN gene contains a dihydrouridine arm replacement loop, as is the case in all other metazoan mtDNAs examined to date. AGA and AGG are found in theF. hepatica mitochondrial protein genes and both codons appear to specify serine. These findings concerningF. hepatica mtDNA indicate that both a dihydrouridine arm replacement loop-containing tRNAserAGN gene and the use of AGA and AGG codons to specify serine must first have occurred very early in, or before, the evolution of metazoa.