Required for the formation of a threonylcarbamoyl group on adenosine at position 37 (t(6)A37) in tRNAs that read codons beginning with adenine. Catalyzes the conversion of L-threonine, HCO(3)(-)/CO(2) and ATP to give threonylcarbamoyl-AMP (TC-AMP) as the acyladenylate intermediate, with the release of diphosphate. Is also able to catalyze the reverse reaction in vitro, i.e. the formation of ATP from TC-AMP and PPi. Shows higher affinity for the full-length tRNA(Thr) lacking only the t(6)A37 modification than for its fully modified counterpart. Could also be required for the maturation of 16S rRNA. Binds to double-stranded RNA but does not interact tightly with either of the ribosomal subunits, or the 70S particles. ATP + hydrogencarbonate + L-threonine = diphosphate + H2O + L-threonylcarbamoyladenylate kcat is 0.10 sec(-1) for ATP-dependent TC-AMP formation and 20 sec(-1) for the reverse reaction. Interacts with TsaB and TsaD. The knockdown of this gene results in a loss of t(6)A37 modification in tRNAs. TsaBCDE are necessary and sufficient for tRNA(NNU) t(6)A37 threonylcarbamoyladenosine modification in vitro in E.coli. Unlike previously thought, the formation of TC-AMP does not proceed via an ATP-activated HCO(3)(-) intermediate such as carboxy-AMP. Belongs to the SUA5 family. TsaC subfamily. Was also proposed to catalyze the transfer of the threonylcarbamoyl moiety of TC-AMP to the N6 group of A37 (PubMed:21285948 and PubMed:21775474). However, it was shown that this reaction is catalyzed in B.subtilis by the TsaEBD proteins (PubMed:23072323). 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 E.coli 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.