O6-methylguanine (O6meG) lesions of double-stranded DNA have been associated with mutation and neoplastic transformation. These lesions can, in principle, be produced by at least three different mechanisms: direct alkylation of G X C base pairs in double-stranded DNA; alkylation of guanine residues in single-stranded regions of DNA associated with replication forks; and alkylation of the DNA precursor pool followed by incorporation of O6-methyl deoxyguanosine triphosphate (O6-medGTP) during DNA replication. DNA biosynthesis subsequent to all three events will generate predominantly O6-meG X T base pairs as O6meG preferentially pairs with T. We show here that O6meG X T base pairs are mutagenic; that transalkylase repair has a direct role in the generation of mutations induced by alkylated pool nucleotides; and that the Escherichia coli mismatch repair system is capable of repairing mutagenic G X T intermediates.

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The interaction of several synthetic analogues of d-ApA with Poly U and Poly dT was examined to explore the effects of substituents at phosphorus on binding properties of oligonucleotides. These analogues contained a bulky, lipophilic group (2,2 ,2-trichloroethoxy or 2,2 ,2-trichloro-1, l-diimethylethoxy) a small, uncharged hydrogen-bonding group (amido), or a cationic phosphoramidate (2-aminoethylamido, protonated in neutral aqueous media) in place of the anionic oxygen of the internucleotide phosphate. As determined by "melting curves" each formed a complex with Poly U more stable than the Poly U-d-ApA complex. Binding to Poly dT was comparable or in some cases stronger. Checks on composition (mixing curves) revealed the expected stoichiometry of ldA:2U (or 2dT). Stereochemistry at phosphorus influenced stability of the complexes, but the effect was not a major one. These results suggest that oligonucleotides containing large, lipophilic groups, as well as small non-ionic groups (e.g., the methyl phosphonates) or polar groups, could be useful as probes in hybridization experiments. INTRODUCTIONDefined oligonucleotides possessing pendant groups linked covalently to specified phosphorus atoms in the backbone chain have potential as tools in molecular biology. The pendant groups could serve as fluorescent markers, as signals to stop enzymatic reactions at selected sequences, as lipophilic centers to enhance interaction with membranes, as factors that stabilize hybridization, and as sites for triggering cross-linking reactions. Pioneering studies with small pendant groups (methyl and ethyl esters and methyl

Base modification during solid-phase phosphoramidite synthesis of oligodeoxynucleotides has been investigated. We have discovered chemical modification that converts dG and dG-containing oligomers to a fluorescent form. This modification has been linked to N,N-dimethylaminopyridine (DMAP), an acylation catalyst, which can displace phosphate triester adducts at the 6-position of guanine. Further, we have found that this fluorescent intermediate can be converted in ammonium hydroxide solution to 2,6 diaminopurine deoxyribonucleoside (2,6 DAP), a potentially mutagenic nucleoside analog. We have shown that N-methylimidazole (NMI) in place of DMAP eliminates the fluorescent species and reduces 2,6 DAP contamination.