Novel amber suppressor tRNAs of mammalian origin

Two amber suppressor tRNAs have been isolated from calf liver. They are different from previously identified naturally occurring amber suppressors of eukaryotes in so far as they are neither tRNATyr nor tRNAGln. They are leucine iso-acceptors and their nucleotide sequence indicates that they harbour a CAA and a CAG anticodon respectively. Both species are functional as amber suppressors as demonstrated by readthrough of the amber codon which terminates the 126 kd protein gene of tobacco mosaic virus RNA. The results bring new information in the discussion of codon-anticodon recognition and regulation of termination in eukaryotic protein synthesis.