A NOVEL WOBBLE RULE FOUND IN STARFISH MITOCHONDRIA - PRESENCE OF 7-METHYLGUANOSINE AT THE ANTICODON WOBBLE POSITION EXPANDS DECODING CAPABILITY OF TRANSFER-RNA

In the starfish mitochondrial (mt) genome, codons AGA and AGG (in addi tion to AGU and AGC) have been considered to be translated as serine. There is, however, only a single candidate mt tRNA gene responsible fo r translating these codons and it has a GCT anticodon sequence, but gu anosine at the first position of the anticodon should base pair only w ith pyrimidines according to the conventional wobble rule. To solve th is enigma, the mt tRNA(GCU)(ser) was purified, and sequence determinat ion in combination with electrospray Liquid chromatography/mass spectr ometry revealed that 7-methylguanosine is located at the first positio n of the anticodon. This is the first case in which a tRNA has been fo und 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 be en generated by base modification in the tRNA anticodon.