Reaction of O -methylhydroxylamine with adenosine shifts tautomeric equilibrium to cause transitions

Crystallographic studies on damaged DNAs. I. An N(6)-methoxyadenine residue forms a Watson-Crick pair with a cytosine residue in a B-DNA duplex

Oxyamines such as hydroxylamine and methoxylamine disturb DNA replication and act as potent mutagens, causing nucleotide transition from one purine to another or one pyrimidine to another. In order to investigate mismatch base-pairing in DNA damaged with oxyamines, a dodecamer with the sequence d(CGCGmo(6)AATCCGCG), where mo(6) A is 2'-deoxy-N(6)-methoxyadenosine, was synthesized and its crystal structure determined. No significant conformational changes are found between the present dodecamer and the original undamaged B-form dodecamer. Electron density maps clearly show that the mo(6)A residue forms a base-pair with a 2'-deoxycytidine residue through hydrogen bonds similar to a Watson-Crick G.C base-pair. For these hydrogen bonds to be made, N(6)-methoxyadenine must chemically take the imino form. The methoxylation thus enables the adenine base to mimic a guanine base. As a result, misincorporation of 2'-deoxycytidine instead of thymidine, or 2'-deoxyadenosine instead of 2'-deoxyguanosine, can occur in DNA replication.

Crystallographic studies on damaged DNAs. II. N(6)-methoxyadenine can present two alternate faces for Watson-Crick base-pairing, leading to pyrimidine transition mutagenesis

In a previous paper, 2'-deoxy-N(6)-methoxyadenosine (mo(6)A) was shown to form a mismatch base-pair with 2'-deoxycytidine with a Watson-Crick-type geometry. To fully understand the structural basis of genetic mutations with damaged DNA, it is necessary to examine whether the methoxylated adenine residue still has the ability to form the regular Watson-Crick pairing with a thymine residue. Therefore, a DNA dodecamer with the sequence d(CGCGmo(6)AATTCGCG) has been synthesized and its crystal structure determined. The methoxylation has no significant effect on the overall DNA conformation, which is that of a standard B-form duplex. The methoxylated adenine moieties adopt the amino tautomer with an anti conformation around the C(6)-N(6) bond to the N(1) atom, and they form a Watson-Crick base-pair with thymine residues on the opposite strand, similar to an unmodified adenine residue. It is concluded that methoxylated adenine can present two alternate faces for base-pairing, thanks to the amino<-->imino tautomerism allowed by methoxylation. Based on this property, two gene transition routes are proposed.

In vivo formation and persistence of modified nucleosides resulting from alkylating agents

Alkylating agents are ubiquitous in the human environment and are continuously synthesized in vivo. Although many classes exist, interest has been focused on the N-nitroso compounds, since many are mutagens for bacteria, phage, and cells, and carcinogens for mammals. In contrast to aromatic amines and polyaromatic hydrocarbons which can react at carbons, simple alkylating agents react with nitrogens and oxygens: 13 sites are possible, including the internucleotide phosphodiester. However, only the N-nitroso compounds react extensively with oxygens. In vivo, most possible derivatives have been found after administration of methyl and ethyl nitroso compounds. The ethylating agents are more reactive toward oxygens than are the methylating agents and are more carcinogenic in terms of total alkylation. This is true regardless of whether or not the compounds require metabolic activation. It has been hypothesized that the level and persistence of specific derivatives in a "target" cell correlates with oncogenesis. However, no single derivative can be solely responsible for this complex process, since correlations cannot be made for even a single carcinogen acting on various species or cell types. Some derivatives are chemically unstable, and the glycosyl bond is broken (3- and 7-alkylpurines), leaving apurinic sites which may be mutagenic. These, as well as most adducts, are recognized by different enzymatic activities which remove/repair at various rates and efficiencies depending on the number of alkyl derivatives, as well as enzyme content in the cell and recognition of the enzyme. Evaluation of human exposure requires early and sensitive methods to detect the initial damage and the extent of repair of each of the many promutagenic adducts.

Fidelity in transcription assessed by DNA-dependent RNA polymerases

The use of in vitro transcription of polynucleotides by prokaryotic and eukaryotic DNA-dependent RNA polymerases has proven of great usefulness in detecting mutagenic base modifications. Quite similar results are obtained with ribo- and deoxyribopolymers as templates, and with Mg2+ and Mn2+ at optimal concentrations. Recent studies have centered on polymers containing etheno C, as well as its hydrated form. These modifications are of particular interest, since they are the consequence of metabolised vinyl chloride, a proven human carcinogen.