Required for the formation of a threonylcarbamoyl group on adenosine at position 37 (t(6)A37) in tRNAs that read codons beginning with adenine. Is involved in the transfer of the threonylcarbamoyl moiety of threonylcarbamoyl-AMP (TC-AMP) to the N6 group of A37, together with TsaE and TsaB; this reaction does not require ATP in vitro. TsaD likely plays a direct catalytic role in this reaction. adenosine(37) in tRNA + L-threonylcarbamoyladenylate = AMP + H(+) + N(6)-L-threonylcarbamoyladenosine(37) in tRNA Binds 1 Fe(2+) ion per subunit. May form a heterodimer with TsaB. The four proteins YwlC, TsaD, TsaB and TsaE are necessary and sufficient for tRNA(NNU) t(6)A37 threonylcarbamoyladenosine biosynthesis in vitro in B.subtilis. Belongs to the KAE1 / TsaD family. The well-known t(6)A modification appears to be a hydrolyzed artifact of natural cyclic t(6)A (ct(6)A) that occurs during the preparation and handling of tRNA in B.subtilis and many other species (PubMed:23242255). In these species, the t(6)A modification is processed further by dehydration into ct(6)A, a reaction catalyzed by TcdA.