Transfer RNA methylase activities of SV40-transformed cells and cells infected with animal viruses.

INTRODUCTION The possible induction of new transfer RNA-methylating enzymes by the tumor-producing virus, Simian Virus 40 (SV4O), and by other DNA-containing animal viruses has been studied in connection with the aberrant methylation hypothesis of vital carcinogenesis. The tRNA methylase â€oeactivity†• and the tRNA methylase â€oecapacity†• were measured after productive infection of African green monkey kidney (CV-l) cells by SV4O, vaccinia, and herpes simplex viruses. The methylase â€oeactivity†• and the methylase â€oecapacity†• were also studied after abortive infection of primary mouse kidney cultures by SV4O. Methylase â€oeactivity†• was assayed with limiting amounts of enzyme and with excess tRNA (yeast, Escherichia coli, Bacillus subtiis, and rabbit liver). Methylase â€oecapacity†• was assayed in the presence of limiting amounts of tRNA but with excess enzyme. Neither the tRNA methylase â€oeactivity†• nor the tRNA methylase â€oecapacity†• was enhanced by infecting cells with any of three DNA-containing viruses. However, control experiments with the same cell extracts confirmed the fact that all three viruses did induce thymidine kinase activity. Although the tRNA methylases were not enhanced in virus-infected cells, both the tRNA methylase â€oeactivity†• and the tRNA methylase â€oecapacity†• were increased about 2-to 4-fold in various mouse kidney cell lines transformed by SV4O. To learn whether the increases were dependent on the metabolic state of the transformed cells, we studied the tRNA methylases at various times after subculture of the cells. The changes in the tRNA methylase during growth were relatively small compared with changes in thymidine kinase activity. Unlike tRNA methylase, thymidine kinase increased markedly in activity during exponential cell growth and then declined sharply as the cells entered the stationary growth phase. Thus, tRNA methylase fluctuations during cell growth probably do not account for the high levels of tRNA methylase in SV4O-transformed cells. The enhanced tRNA methylases of SV4O-transformed cells have been interpreted as indicative of a generalized derepression of host cell functions related to growth rather than the induction of a new tRNA methylase controlled directly by SV4O genes. Induction of new tRNA-methylating enzymes by tumor-producing viruses has been suggested by Tsutsui et a!. (27) as one possible cause of tumor induction by viruses. If methylating enzymes were induced, aberrant methylation of host nucleic acids could occur, just as with the carcinogenic alkylating agents. Should the methyl groups in tRNA prove to be pivots on which some regulatory mechanism of protein synthesis hinges, then …