The methylation of transfer RNA in Escherichia coli

Pleiotrophic effects of point mutations in yeast tRNA(Asp) on the base modification pattern

The base-modification pattern has been studied in several synthetic variants of yeast tRNA(Asp) injected into Xenopus laevis oocytes. Certain point mutations in the D-stem and the variable loop of the tRNA led to considerably decreased levels of m1G37, psi 40 and Q34/manQ34 in the anticodon stem or loop and an increased rate of synthesis for m5C49 in the T-stem. The formation of m2G6 in the aminoacyl-stem was not affected in any of the tRNA-variants. Thus, mutations in one part of the tRNA-molecule can have long-range effects on the interactions between another part of the tRNA and the tRNA modifying enzymes.

Effects of DL-ethionine on mouse liver tRNA base composition

Treatment of mice with DL-ethionine and adenine causes a reduction of all methylated bases of liver tRNA. This effect is dose-dependent and specific for the methylated bases. Individual methylated components are affected to different extents, m22G being most sensitive to inhibition.

The methylation of tRNA

The methylation of tRNA is a post-transcriptional modification which is achieved by specific enzymes, the tRNA methylases, with S adenosylmethionine as a methyl donor. The level and pattern of methylation are characteristic of the tRNA species and origin. Abnormally methylated tRNAs have been obtained, in vivo and in vitro, by a variety of methods, and their properties have been studied. The tRNA methylases are found in all cells and tissues. Their activity varies with the differentiation state of the cells, and under the influence of many internal and external factors ; it is especially elevated in embryonic and cancerous tissues. These enzymes are very unstable, and none of them has been purified to homogeneity. We present here their known properties and we propose a theory concerning their specificity. Finally, after reviewing the few available experimental data, we discuss the current hypotheses and speculations about the roles and functions of tRNA methylation.