Position-dependent effects of regioisomeric methylated adenine and guanine ribonucleosides on translation

Reversible methylation of the N6 or N1 position of adenine in RNA has recently been shown to play significant roles in regulating the functions of RNA. RNA can also be alkylated upon exposure to endogenous and exogenous alkylating agents. Here we examined how regio-specific methylation at the hydrogen bonding edge of adenine and guanine in mRNA affects translation. When situated at the third codon position, the methylated nucleosides did not compromise the speed or accuracy of translation under most circumstances. When located at the first or second codon position, N1-methyladenosine (m1A) and m1G constituted robust blocks to both Escherichia coli and wheat germ extract translation systems, whereas N2-methylguanosine (m2G) moderately impeded translation. While m1A, m2G and N6-methyladenosine (m6A) did not perturb translational fidelity, O6-methylguanosine (m6G) at the first and second codon positions was strongly and moderately miscoding, respectively, and it was decoded as an adenosine in both systems. The effects of methylated ribonucleosides on translation could be attributed to the methylation-elicited alterations in base pairing properties of the nucleobases, and the mechanisms of ribosomal decoding contributed to the position-dependent effects. Together, our study afforded important new knowledge about the modulation of translation by methylation of purine nucleobases in mRNA.