Figure 1.

The bacterial t⁶A biosynthesis cycle, shown here for T. maritima. tRNA and ATP-bound TsaE bind to separate sites on the TmTsaB₂D₂ heterocomplex (1). TC–AMP, synthesized by TsaC2, is then delivered to the active site of the tRNA-bound TsaD subunit (2), and the threonylcarbamoyl (TC) moiety is transferred to A₃₇ of tRNA, forming t⁶A (3). Following dissociation of the t⁶A-modified tRNA from the complex (4), ATP hydrolysis resets the system to the initial state and TsaE is released (5), thus allowing the next substrate tRNA to bind for the next cycle. The two halves of the complex are reset and utilized in alternating cycles. Halos indicate a pre-‘reset’ state for a given half.