Isolation and comparison of ribothymidine-lacking tRNAs of fetal, newborn and adult bovine tissues.

Although ribothymidine (rT) is the most common methylated nucleoside in tRNA, a wide variety of bovine tissues have now been found to contain a class of tRNAs which totally lack rT and have an unmodified uridine in its place. The tissues studied include bovine brain, kidney, liver, thymus and testicles from adult, newborn and fetal stages. The class of tRNA was detected by its ability to be methylated with Escherichia coli rT-forming uracil methylase with radioactive S-adenosyl-L-methionine as the methyl donor. In each case rT was shown to account for at least 95% of the methylated products produced. In vitro methylated tRNA populations were compared by fractionation of double-labeled tRNAs on RPC-5 columns. Three major methyl-accepting tRNA peaks were found for all mammalian tissues studied. The level of methyl acceptance in these peaks was found to vary considerably between tRNAs of different tissues. A major difference in the methyl-accepting tRNA populations of bovine liver and calf thymus was observed. Little similarity was found in the rT-lacking class of tRNAs of bovine liver and wheat germ. Three members of the rT-lacking class of bovine liver tRNA were isolated and found to be two species of valine tRNA and one species of threonine tRNA. All three tRNA's completely lacked rT and could be quantitatively methylated with E. coli uracil methylase.