Transfer RNA (Gm18) methyltransferase (TrmH, EC 188.8.131.52) catalyzes the transfer of a methyl group from S-adenosyl-L-methionine (AdoMet) to the 2'-OH of the ribose of guanosine at position 18 in the D-loop of tRNA. The methyl group donor, AdoMet, is converted to S-adenosyl-L-homocysteine (AdoHcy) in the reaction. Recently, we determined the crystal structures of the AdoMet-bound and apo-forms of TrmH from Thermus thermophilus HB8. Based on the crystal structures, we carried out site-directed mutagenesis to clarify the functions of the conserved amino acid residues among SpoU RNA methyltransferases. During the course of the study, we found that the subunit structure of the wild-type TrmH changed according to the presence or absence of AdoMet analogues. In the absence of AdoMet or AdoHcy, the enzyme forms a homodimer. However, the addition of AdoMet induced the formation of a tetramer structure. Furthermore, the addition of AdoHcy caused the dissociation of subunits and gave rise to a monomer structure. These findings suggest that the dissociation of tRNA from the tRNA-enzyme complex after the methyl-transfer reaction is caused by the dissociation of enzyme subunits. In this meeting, we discuss the relationship between the catalytic mechanism and the change of the subunit structure of TrmH.
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