tRNA (adenine-N1)-methyltransferase from Dictyostelium discoideum. Purification, characterization and developmental changes in activity

An enzyme activity transferring methyl groups from S-adenosylmethionine to endogenous tRNA was detected in the cytosol of aggregative Dictyostelium discoideum amoebae. This enzyme was purified more than 1000-fold and was characterized as a tRNA (adenine-N1-)-methyltransferase. Kinetic analysis yielded a K0.5 for S-adenosylmethionine of 0.27 microM and competitive inhibition by S-adenosylhomocysteine showed an I0.5 of 0.26 microM. The tRNA methyltransferase activity was stimulated by monovalent cations and the pH optimum was 7.3. tRNAs isolated from D. discoideum as well as from other eucaryotic sources could be methylated only to a minor extent. In contrast, Escherichia coli tRNA accepted up to 0.6 mol methyl group/mol tRNA, suggesting that the target nucleotide is unmethylated in procaryotic tRNA, but is commonly methylated in tRNAs from eucaryotic organisms. The activity of the methyltransferase increased 4-6-fold during cell differentiation from the vegetative to the aggregative stage.