The Contrast between the Methylation of Transfer Ribonucleic Acid in Vivo and in Vitro by Normal and SV40 Transformed 3T3 Cells

The methylation of tRNA in vitro and in vivo was studied using normal mouse cells (3T3) and transformed mouse cells (SV3T3) which were obtained by transformation of the normal 3T3 cells with SV40 virus. Cytoplasmic extracts prepared from transformed cells transferred methyl groups in vitro from S-adenosylmethionine to either Escherichia coli B or mouse tRNA to a much greater extent than did extracts of nontransformed control cells. Furthermore, the tRNA methylating enzymes of the transformed cells had an altered specificity which resulted in methylation of new sites on E. coli B tRNA in addition to those methylated by nontransformed cell enzymes. It was also demonstrated that E. coli B tRNA methylated by transformed cell extracts chromatographed differently on benzoylated DEAE-cellulose and had more methylated oligonucleotide sequences than did E. coli B tRNA methylated by normal cell extracts. In marked contrast to the results obtained in vitro with mouse enzymes and E. coli B tRNA, no differences were detected in tRNA which was methylated in vivo by growing normal and transformed cells in l-[methyl-14C]methionine or l-[methyl-3H]methionine. The extent of methylation and the methylated base composition of the two tRNA populations were the same. The normal and transformed cell tRNA co-chromatographed on benzoylated DEAE-cellulose and had the same methylated oligonucleotide sequences.