Sites of alkylation of poly(U) by agents of varying carcinogenicity and stability of products.

Several alkylating agents of widely varying reported carcinogenicity (dimethylsulfate, diethylsulfate, ethylmethanesulfonate, methylnitrosourea, ethylnitrosourea and ethylnitrosoguanidine) were reacted with poly(U) at pH values ranging from 4.5 to 7.5. All nucleophilic centers (internal phosphate groups, ribose hydroxyls, and O2, N-3 and O4 sites of the uracil base) were found reactive, though to different extents, at neutrality and in slightly acid solution. The distribution of products is a function of the alkylating agent and pH. The nitroso compounds are more reactive toward oxygens than are dialkylsulfates and alkylalkanesulfonates. The ratio of N : O alkyl products is strongly pH dependent, primarily due to the N-3 being most reactive at the higher pH values, while the diester is most reactive at the lower pH values. The extent of reaction of the O2, O4 or 2'-O or ribose is not greatly affected over the pH range tested. At pH 5.0 alkyl ribophosphotriesters mainly lose alchol to re-form a stable phosphodiester. With increasing OH- concentration, the favored reaction is chain scission at the 3'-O-P bond.     

Comparison of the relative mutagenicities of O-alkylthymines site-specifically incorporated into phi X174 DNA

The relative mutagenicities of O-alkylthymine-DNA adducts were analyzed in vivo by site-specific mutagenesis. Purified DNA polymerases were used to incorporate O4-methyl (Me)-, O4-ethyl (Et)-, O4-isopropyl (iPr)-, or O2-Me-dTTP onto the 3' terminus of a synthetic oligonucleotide (15-mer) hybridized to phi X174 am3 DNA. The product oligonucleotides were further extended in the presence of unmodified dNTPs to yield 21-mers containing single O-alkylthymine adducts opposite the adenine residue of the bacteriophage amber codon. Polyacrylamide gel electrophoresis and nearest-neighbor analyses confirmed the identities and nucleotide positions of the adducts. Transfection and replication of the site-specifically alkylated DNAs in ada- Escherichia coli (defective in the alkyltransferase capable of repairing O4-alkylthymine-DNA adducts) yielded mutant progeny phage with reversion frequencies of: O4-Me-dThd (19.5 X 10(-6) ) greater than O4-Et-dThd (7.5 X 10(-6) ) greater than O4-iPr-dThd (3.0 X 10(-6) ) greater than or equal to O2-Me-dThd (1.0 X 10(-6) ) approximately equal to dThd (2.0 X 10(-6) ). None of the adducts produced mutations above background following replication in ada+ E. coli. DNA sequence analyses of 40 independently isolated mutant phage derived from the O4-Me- or O4-Et-dThd-containing DNAs showed that all mutants contained guanine residues opposite the original site of the alkylthymines. These data are consistent with a mechanism of mutagenesis involving the formation of O4-alkyl-T.G base pairs during DNA replication in E. coli and suggest that the formation of A.T----G.C transition mutations is characteristic of mutagenesis by O4-Me- and O4-Et-dThds in vivo.