A thermostable tRNA (guanosine-2')-methyltransferase from Thermus thermophilus HB27 and the effect of ribose methylation on the conformational stability of tRNA.

An S-adenosylmethionine-=dependent tRNA (guanosine-2'-)-methyltransferase (EC 2.1.1.34) was purified to the homogeneous state (2,400-fold) from a cell-free extract of an extreme thermophile, Thermus thermophilus HB27. The enzyme was highly resistant to heat as reported for other enzymes from thermophilic organism. The enzyme is monomeric and its molecular weight was estimated to be about 20,000. The Km values for S-adenosylmethionine and for Escherichia coli tRNAPhe were determined to be 0.47 microM and 10 nM, respectively, while the Ki for a competitive inhibitor S-adenosylhomocysteine, was 1.67 microM. When yeast tRNAPhe was methylated with the purified Gm-methyltransferase, a stoichiometric amount of methyl group was incorporated into the invariant guanosine at position 18 in the D-loop. Yeast tRNAPhe and E. coli tRNAMet, which were quantitatively methylated with the enzyme, were very similar to the native tRNAs with regard to amino acid acceptor activity and melting temperature, but were more resistant to RNase T1 and RNase A digestions than the corresponding native tRNAs.