A NOVEL MODIFIED NUCLEOSIDE FOUND AT THE FIRST POSITION OF THE ANTICODON OF METHIONINE TRANSFER-RNA FROM BOVINE LIVER-MITOCHONDRIA

Methionine tRNA was purified from bovine liver mitochondria, and its n ucleotide sequence was determined. The tRNA possesses only three postt ranscriptionally modified nucleosides, two pseudouridines in the antic odon and T stems and a previously unknown nucleoside specified by the gene sequence as cytidine, in the first position of the anticodon. Str ucture analysis of the anticodon nucleoside by mass spectrometry revea led a molecular mass 28 Da greater than that of cytidine, and unmodifi ed ribose, with substitution at C-5 implied by hydrogen-deuterium exch ange experiments. Proton NMR of the intact tRNA showed presence of a f ormyl moiety, thus leading to the candidate structure 5-formylcytidine (f(5)C), not a previously known compound. The structure assignment wa s confirmed by chemical synthesis and comparison of data from combined HPLC/mass spectrometry and proton NMR for the natural and synthetic n ucleosides. The potential function of f(5)C in the tRNA(Met) anticodon is discussed with regard to codon-anticodon interactions.