Methyl-accepting RNA in 13762 Mammary Adenocarcinoma Correlated with Low Adenine Methyltransferase Levels

Methylation reactions carried out with mammalian transfer RNA (tRNA) methyltransferases and RNA prepared from the homologous source do not normally show significant incorporation of methyl groups into the tRNA. However, our studies with the transplantable mammary adenocarcinoma 13762 indicate that tRNA from this tumor can be methylated in vitro with the homologous methyltransferases at a level 10 times higher than seen when tRNA from rat liver is reacted with its own enzyme. Analysis of the methylated nucleosides formed in vitro shows that greater than 80% of the methyl groups incorporated into 13762 RNA is found as 1-methyladenosine. Examination of the tRNA methyltransferase content of adenocarcinoma 13762 indicates that this tumor possesses unusually low levels of the adenine-1 methyltransferase responsible for methylating the invariant adenine at position 58 on tRNA. The nature of the methyl-accepting RNA from 13762 tumors has been examined using highly purified adenine-1 methyltransferase prepared from rat liver. Methylation of tumor RNA eluted from polyacrylamide gels after separation by electrophoresis indicated that while methyl-accepting material is found throughout the RNA-containing region of the gel, RNAs migrating slower than the bulk of mature tRNA are particularly good substrates for adenine methyltransferase. Similarly, when 13762 RNA is first methylated by the adenine-methylating enzyme and then run on acrylamide gels, several peaks of methyl -3H are seen in the region of slow-migrating tRNA. These results indicate that the 1-methyladenine deficiency in adenocarcinoma 13762 results in the appearance of selected populations of tRNA which are substrates in vitro for adenine-1 methyltransferase. The electrophoretic mobility of the methylaccepting RNA in 13762 adenocarcinomas suggests that at least some of these may be precursor tRNAs.