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. (C) 2002 Elsevier Science Ltd. All rights reserved.