The naturally occurring N6-threonyl adenine in anticodon loop of Schizosaccharomyces pombe tRNAi causes formation of a unique U-turn motif

Modified nucleosides play an important role in structure and function of tRNA. We have determined the solution structure of the anticodon stem–loop (ASL) of initiator tRNA of Schizosaccharomyces pombe. The incorporation of N6-threonylcarbamoyladenosine at the position 3′ to the anticodon triplet (t⁶A37) results in the formation of a U-turn motif and enhances stacking interactions within the loop and stem regions (i.e. between A35 and t⁶A37) by bulging out U36. This conformation was not observed in a crystal structure of tRNAi including the same modification in its anticodon loop, nor in the solution structure of the unmodified ASL. A t⁶A modification also occurs in the well studied anti-stem–loop of lys-tRNA_UUU. A comparison of this stem–loop with our structure demonstrates different effects of the modification depending on the loop sequence.