S-adenosyl-L-methionine-dependent methyltransferase responsible for the addition of the methyl group in the formation of N6-methyl-N6-threonylcarbamoyladenosine at position 37 (m(6)t(6)A37) of the tRNA anticodon loop of tRNA(Ser)(GCU). The methyl group of m(6)t(6)A37 may improve the efficiency of the tRNA decoding ability. May bind to tRNA. N(6)-L-threonylcarbamoyladenosine(37) in tRNA + S-adenosyl-L-methionine = H(+) + N(6)-methyl,N(6)-L-threonylcarbamoyladenosine(37) in tRNA + S-adenosyl-L-homocysteine May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the tRNA methyltransferase O family.