An unnatural base pair between imidazolin-2-one and 2-amino-6-(2-thienyl)purine in replication and transcription.

Nucleosides of imidazolin-2-one (designated by z) were designed and synthesized as pairing partners of 2-amino-6-(2-thienyl)purine (designated by s). Previously, we developed an unnatural base pair between s and pyridine-2-one (designated by y), and polymerases specifically incorporated the substrate of y into DNA and RNA opposite s in templates. Although s was efficiently incorporated opposite y, A was also incorporated opposite y with high efficiency. The replacement of y by z effectively improved the incorporation selectivity of s. The incorporation efficiency of A opposite z decreased, but the efficient incorporation of s opposite z was maintained as compared to that of the s-y pairing.