A Novel Wobble Rule Found in Starfish Mitochondria

In the starfish mitochondrial (mt) genome, codons AGA and AGG (in addition to AGU and AGC) have been considered to be translated as serine. There is, however, only a single candidate mt tRNA gene responsible for translating these codons and it has a GCT anticodon sequence, but guanosine at the first position of the anticodon should base pair only with pyrimidines according to the conventional wobble rule. To solve this enigma, the mt tRNA GCUser was purified, and sequence determination in combination with electrospray liquid chromatography/mass spectrometry revealed that 7-methylguanosine is located at the first position of the anticodon. This is the first case in which a tRNA has been found to have 7-methylguanosine at the wobble position. It is suggested that methylation at N-7 of wobbling guanosine endows the tRNA with the capability of forming base pairs with all four nucleotides, A, U, G, and C, and expands the repertoire of codon-anticodon interaction. This finding indicates that a nonuniversal genetic code in starfish has been generated by base modification in the tRNA anticodon.