Figure 2.

A) Selected examples of extensive RNA nucleobase modification identified in biological nucleic acids. i) Adenine is most often alkylated at its N6 amino group, but here it has also been altered at the C2 position with an alkylthio group. ii) Guanine has been heavily reworked by the replacement of its heterocyclic N7 with a carbon, which is further substituted with an amidine group. iii) Cytosine has been tautomerized and aminated at the C1 position with a lysine residue. iv) Uracil substituted at the C5 position with a secondary aminoalkyl group and a thiocarbonyl at C2. B) Selected examples of conservative base modifications that are known to occur more often in RNA. The modified pyrimidines, with the exception of s²U, are also utilized in exotic DNA genomes. Frequently, the modifications include alkylation of the purine exocyclic amino groups (as for N2 of guanine and N6 of adenine) or modifications at C5 of the pyrimidines. Also shown is the deamination of adenine leading to hypoxanthine (called inosine in RNA), an important modification used as a guanine surrogate in edited RNA transcripts.