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 methyl-accepting
RNA in 13762
adenocarcinomas suggests that at least some of these may be precursor tRNAs.