Osmium Tag for Post-transcriptionally Modified RNA.

5-Methylcytidine (m5 C) and 5-methyluridine (m5 U) are highly abundant post-transcriptionally modified nucleotides that are observed in various natural RNAs. Such nucleotides were labeled through a chemical approach, as both underwent oxidation at the C5=C6 double bond, leading to the formation of osmium-bipyridine complexes, which could be identified by mass spectrometry. This osmium tag made it possible to distinguished m5 C and m5 U from their isomers, 2'-O-methylcytidine and 2'-O-methyluridine, respectively. Queuosine and 2-methylthio-N6 -isopentenyladenosine in tRNA were also tagged through complex formation.