INVITRO MISPAIRING SPECIFICITY OF O2-ETHYLTHYMIDINE

The O2-position of thymine is a major site of base alkylation by N-nitroso-alkylating agents, and its biological relevance remains obscure. The potential significance of this DNA damage was ascertained by studying in vitro DNA replication properties of O2-ethylthymidine (O2-Et-dT) site-specifically incorporated into a 36-nucleotide template. DNA replication was initiated eight nucleotides away from the O2-Et-dT lesion by Escherichia coli polymerase I (Klenow fragment) using a 17-nucleotide primer. In the presence of 10-mu-M dNTP and Mg2+, O2-Et-dT blocked DNA replication predominantly (94%) 3' to O2-Et-dT, with the remainder (5%) blocked after incorporation of a nucleotide opposite O2-Et-dT (incorporation-dependent blocked product). Postlesion synthesis was negligible (< 1%). Nucleotide incorporation opposite O2-Et-dT increased to 23% at 200-mu-M dNTP. Postlesion synthesis remained negligible (< 2%). DNA sequencing revealed dA present opposite O2-Et-dT in the incorporation-dependent blocked product. Negligible postlesion synthesis suggests that incorporation of dA opposite O2-Et-dT inhibits in vitro DNA synthesis. The O2-Et-dT.dA base pair may also impede DNA synthesis in vivo, contributing to the cytotoxicity of the ethylating agents. Substitution of Mn2+ for Mg2+ enhanced nucleotide incorporation opposite O2-Et-dT and produced postlesion synthesis (16%) at 10-mu-M dNTP, which increased to 39% at 200-mu-M dNTP. DNA sequence analysis showed that while dA was present opposite O2-Et-dT in the incorporation-dependent blocked product, both dA and dT were present opposite this lesion in the postlesion synthesis product. The results indicate that when dT is incorporated opposite O2-Et-dT, the nascent chains containing the O2-Et-dT-dT base pair at the 3'-terminus are efficiently extended. The studies suggest a role for O2-Et-dT in A.T --> T.A transversion mutagenesis by ethylating agents.