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 Heterotetramer; composed of two copies of TRM6 and two copies of TRM61. Belongs to the class I-like SAM-binding methyltransferase superfamily. TRM61 family.