Methylation of purified transfer RNAs by liver extracts from normal and mestranol-treated rats

1. tRNA methylases from rat liver were studied with the aid of purified tRNA preparations from Escherichia coli and yeast as substrates. Methylation required S-adenosylmethionine and a cation which could be Mg²⁺, NH₄⁺ or a polyamine. The cation required for optimal methylation and the methylated bases produced varied with the tRNA used as a substrate. 2. Rat liver tRNA, tRNA₂^Arg and tRNA₁^Met from yeast were not significantly methylated by the crude liver extracts. Yeast tRNA^Asp was methylated with the formation of N²-methylguanosine at the guanosine residue at position 26 from the 5′-OH end of the molecule. E. coli tRNA^Glu was methylated with the formation of 5-methylcytidine at residue 48 and 1-methyladenosine at residue 58. E. coli tRNA^fMet was methylated with the formation of N²-methylguanosine at position 27, 1-methylguanosine at position 9, 1-methyladenosine at position 59 and 5-methylcytidine at position 49. 3. There were no differences in the specificity of tRNA methylases from livers of rats given large amounts of the steroid hormone mestranol when these were compared to controls although extracts obtained from the mestranol-treated animals showed a greater rate and extent of methylation of the added tRNA.