Nucleic acids are under constant assault from endogenous and environmental agents that alter their physical and chemical properties. O6-methylation of guanosine (m6G) is particularly notable for its high mutagenicity, pairing with T, during DNA replication. Yet, while m6G accumulates in both DNA and RNA, little is known about its effects on RNA. Here, we investigate the effects of m6G on the decoding process, using a reconstituted bacterial translation system. m6G at the first and third position of the codon decreases the accuracy of tRNA selection. The ribosome readily incorporates near-cognate aminoacyltRNAs (aa-tRNAs) by forming m6G-uridine codon-anticodon pairs. Surprisingly, the introduction of m6G to the second position of the codon does not promote miscoding, but instead slows the observed rates of peptide-bond formation by 1000-fold for cognate aa-tRNAs without altering the rates for near-cognate aa-tRNAs. These in vitro observations were recapitulated in eukaryotic extracts and HEK293 cells. Interestingly, the analogous modification N6-methyladenosine (m6A) at the second position has only a minimal effect on tRNA selection, suggesting that the effects on tRNA selection seen with m6G are due to altered geometry of the base pair. Given that the m6G:U base pair is predicted to be nearly indistinguishable from a Watson-Crick base pair, our data suggest that the decoding center of the ribosome is extremely sensitive to changes at the second position. Our data, apart from highlighting the deleterious effects that these adducts pose to cellular fitness, shed new insight into decoding and the process by which the ribosome recognizes codon-anticodon pairs.
- RNA damage