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Comparison of the Specificity and Extent of in Vitro Methylation by Guanylate Residue-specific Transfer RNA Methylases Isolated from Ascites Hepatoma, 3'-Methyl-4-dimethylaminoazobenzene-induced Hepatoma, and Normal Rat Liver¹

The Biology Division, National Cancer Center Research Institute, Chuo-ku, Tokyo, and Japan [S. N.] Cancer Research Institute, Faculty of Medicine, Kyushu University, Fukuoka, Japan [Y. K., H. E.]

The extent and sites of in vitro methylation of guanylate residue-specific tRNA methylases isolated from ascites hepatoma, 3'-methyl-4-dimethylaminoazobenzene-induced hepatoma, and normal rat liver were compared by the use of purified individual Escherichia coli tRNA's as methyl acceptors. The levels of activity of the two methylases, i.e., methylases II and III, were 2- to 4-fold higher in the extract from ascites hepatoma and from 3'-methyl-4-dimethylaminoazobenzene-induced hepatoma than in that from normal rat liver. It was shown that the lower level of methylase activity detected in normal rat liver was not due to the increased amount of methylase inhibitor or RNase. The sites of in vitro methylation of E. coli tRNA^fMet and tRNA by the tumor methylases were the same as those for normal liver; namely, G₂₇ of tRNA^fMet and G₁₀ of tRNA were methylated to provide 2-methylguanosine by methylases II and III, respectively, from tumor cells, as in the case of the methylases from normal liver. These results indicate that specificity of the tRNA methylases, in terms of recognition of location of guanylate residues in tRNA molecules, was not altered in cancerous cells.

¹ This investigation was supported in part by grants from the Princess Takamatsu Cancer Research Fund and the Japanese Ministry of Education.

² Present address: Biology Division, National Cancer Research Institute, Chuo-ku, Tokyo, Japan.

Received 12/17/71. Accepted 2/16/72.