Role of ribothymidine in mammalian tRNAPhe.

We have previously reported that mammalian tRNAsPhe from variation tissues contain different amounts of ribothymidine and that a uridine methylase from Escherichia coli can quantitatively convert these tRNAs to species that contain their full complement ofribothymidine at position 23 from the 3' terminus. The role of ribothymidine in mammalian tRNAs has now been investigated by studying the ability of several highly purified mammalian tRNAsPhe, differing only in their ribothymidine content, to support poly(U)-directed poly(Phe) synthesis under various conditions. Our results indicate that the ribothymidine content of mammalian tRNAPhe can be correlated with the ability of these tRNAs to functionin vitro in a low-magnesium (6 mM), ribosome wash factor-dependent, poly(U)-directed poly(Phe) synthesis system from rat liver. Specifically, the effect of increasing the ribothymidine content in a class C mammalian tRNA becomes manifest in an increased apparent maximum velocity for the overall synthesis of poly(Phe), while the apparent Michaelis constant (Km) remains essentially unchanged. It is postulated that the modifiednucleoside ribothymidine might be involved in the regulation of protein synthesis at the level of translation in mammalian liver.