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. Forms a heterodimer with TsaE. Cells lacking this gene display a slow growth phenotype. Cell size, shape and nucleoid morphology remain unchanged. 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. TsaB subfamily. 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.