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 TsaD and TsaE; this reaction does not require ATP in vitro. TsaB seems to play an indirect role in the t(6)A biosynthesis pathway, possibly in regulating the core enzymatic function of TsaD (By similarity). Homodimer. Structural analysis identified two distinctly different modes of TsaB dimer formation. In one form, prevalent in the absence of nucleotide, the putative nucleotide-binding site is incomplete and lacks a binding pocket for a nucleotide base. In the second form, an interface between the two subunits apparently completes a nucleotide-binding site. This analysis suggests that the two dimer architectures observed in the crystal structures correspond to a free and a nucleotide-bound form of TsaB. This suggests that nucleotide binding to TsaB acts as a regulator or a switch that changes the shape of TsaB and allows it to interact with different partner proteins (PubMed:21858042). Belongs to the KAE1 / TsaD family. TsaB subfamily.