Mutagenesis by N4-aminocytidine: induction of AT to GC transition and its molecular mechanism.

N4-Aminocytidine is a potent mutagen toward Escherichia coli and Salmonella typhimurium. It induced reversion of an amber mutant of phi X174 phage (am3) to the wild type. This reversion was shown to be exclusively due to the AT to GC transition. It is likely that N4-aminocytidine is metabolized within the bacterial cells into N4-aminodeoxycytidine 5'-triphosphate and this nucleotide is incorporated into DNA during the multiplication of the cells and the phages, thereby causing base-pair transitions. The molecular basis for this erroneous replication was obtained in studies of in vitro incorporation of N4-aminodeoxycytidine 5'-triphosphate into polynucleotides catalyzed by the E. coli DNA polymerase I large fragment. The results have shown that this cytosine analogue can be efficiently incorporated as a substitute of cytosine and that it can also be incorporated as a substitute of thymine. The ratio in the rate of the N4-aminocytosine nucleotide incorporation to that of natural nucleotide incorporation was 1/2 to cytosine and 1/30 to thymine. Furthermore, the N4-aminocytosine residues in the polynucleotide templates can be read by the enzyme as efficiently as cytosines, and guanines were incorporated opposite to them.