Deoxynucleoside Triphosphate Containing Pyridazin-3-one Aglycon as a Thymidine Triphosphate Substitute for Primer Extension and Chain Elongation by Klenow Fragments.

Deoxynucleoside 5'-triphosphate was synthesized with 3-oxo-2 H-pyridazin-6-yl (Pz ^O )-a uracil analogue lacking a 2-keto group-as the nucleobase. Theoretical analyses and hybridization experiments indicated that Pz ^O recognizes adenine (A) for formation of a Watson-Crick base pair. Primer extension reactions using nucleoside 5'-triphosphate and the Klenow fragment revealed that the synthetic nucleoside 5'-triphosphate was incorporated into the 3' end of the primer through recognition of A in the template strand. Moreover, the 3'-nucleotide residue harboring Pz ^O as the base was resistant to the 3'-exonuclease activity of Klenow fragment exo+. The primer bearing the Pz ^O base at the 3' end could function in subsequent chain elongation. These properties of Pz ^O were attributed to the presence of an endocyclic nitrogen atom at the position ortho to the glycosidic bond, which was presumed to form an H-bond with the amino acid residue of DNA polymerase for effective recognition of the 3' end of the primer for primer extension. These results provide a basis for designing new nucleobases by combining a nitrogen atom at the position ortho to the glycosidic bond and base-pairing sites for Watson-Crick hydrogen bonding.