Mechanism of mutagenesis induced by cytosine analogs bearing N(4)-substitutions.

The mechanism of mutagenesis induced by dihydropyrimido [4,5-c][1,2]oxazin-7-one deoxyriboside, P-nucleoside, was studied. This analog is highly mutagenic toward Escherichia coli and Salmonella typhimurium. In E. coli, it induces GC-to-AT and AT-to-GC transitions specifically. No transversions are inducible. P-nucleoside was highly mutagenic to a wild-type E. coli, but little mutagenic in a strain lacking thymidine kinase. This indicates that P-nucleoside may be phosphorylated by thymidine kinase after its uptake into bacteria. The mutagenesis induced by P-nucleoside was efficiently inhibited by the addition of thymidine. This inhibition further confirmed the involvement of thymidine kinase in the first step of the metabolism of P-nucleoside in the bacterial cells. These findings indicate that P-nucleoside is a mutagen of a nucleoside-analog type, causing mutations by the erroneous incorporation and replication. The experiments to prove its ambiguous nature in DNA synthesis is now under way.