Synthesis and Structure Elucidation of the Human tRNA Nucleoside Mannosyl-Queuosine

Queuosine (Q) is a structurally complex, non-canonical RNA nucleoside. It is present in many eukaryotic and bacterial species, where it is part of the anticodon loop of certain tRNAs. In higher vertebrates, including humans, two further modified queuosine-derivatives exist - galactosyl-(galQ) and mannosyl-queuosine (manQ). The function of these low abundant hypermodified RNA nucleosides remains unknown. While the structure of galQ was elucidated and confirmed by total synthesis, the reported structure of manQ still awaits confirmation. By combining total synthesis and LC-MS-co-injection, together with a metabolic feeding study of labelled hexoses, we show here that the natural compound manQ isolated from mouse liver deviates from the literature-reported structure. The chemical structure of the natural product manQ features a novel -allyl connectivity. The data reported here shows that the yet unidentified glycosylases that attach galactose and mannose to the Q-base have different constitutional connectivity preferences. Knowing the correct structure of manQ will now pave the way towards further elucidation of its biological function.