A unique unnatural base pair between a C analogue, pseudoisocytosine, and an A analogue, 6-methoxypurine, in replication

Pseudoisocytidine, a C-nucleoside analogue of cytosine, has two possible isomers of the H1- and H3-forms. Enzymatic incorporation experiments confirmed the existence of the two isomers in solution, and the 2′-deoxyribonucleoside triphosphate of pseudoisocytosine (PIC) was incorporated into DNA opposite both guanine and 6-methoxypurine (M) by the Klenow fragment of Escherichia coli DNA polymerase I. In addition to the PIC√M pairing in replication, M also functioned as an A analogue and T was efficiently incorporated opposite M. Thus, the PIC√M pair is regarded as a base pair between a C analogue and an A analogue, and can mediate the interconversion between the G√C and A√T base pairs. The combination of PIC and M could be used as a G√C↔A√T transition mutagen.