Catalytic subunit of tRNA (adenine-N(1)-)-methyltransferase, which catalyzes the formation of N(1)-methyladenine at position 58 (m1A58) in initiator methionyl-tRNA. adenosine(58) in tRNA + S-adenosyl-L-methionine = H(+) + N(1)-methyladenosine(58) in tRNA + S-adenosyl-L-homocysteine Inhibited by calcium and magnesium ions and spermidine. Enhanced by KCl, NaCl and NH(4)Cl in concentrations from 0.1-0.25 M. Concentrations of more than 0.3 M are inhibitory. Optimum pH is 7.3. Heterotetramer; composed of two copies of trmt6 and two copies of trmt61a. Belongs to the class I-like SAM-binding methyltransferase superfamily. TRM61 family.