Endonuclease-resistant apyrimidinic sites formed by neocarzinostatin at cytosine residues in DNA: evidence for a possible role in mutagenesis.

Abstract

When defined-sequence DNA from the lacI region of plasmid pMC1 was treated with the nonprotein chromophore of neocarzinostatin in the presence of various thiols, the predominant lesions were direct strand breaks, occurring primarily at thymine and adenine residues. In the presence of glutathione, however, alkali-dependent strand breaks, occurring at certain cytosine residues, were also detected but were virtually absent when other thiols were used. Chromophore-induced release of free cytosine base from[3H]cytosine-labeled DNA was 2-to 3-fold greater with glutathione than with the other thiols. These results suggest that the alkali-dependent strand break is some form of apyrimidinic site. These sites were substrates for endonuclease IV of Escherichia coli, although a 5-fold greater concentration of enzyme was required for their cleavage than was required for cleavage of apurinic sites in depurinated DNA. These sites were also less sensitive to E. coli endonuclease VI (exonuclease III) by a factor of at least 5 and less sensitive to E. coli endonuclease III by a factor of at least 10. These and other results suggest that these sites are chemically different from normal apurinic/apyrimidinic sites. When chromophore-induced apyrimidinic sites were quantitated as alkali-dependent breaks at 11 specific sites in the lacI gene, a correlation was found between occurrences of these lesions and the reported frequencies of G-C to A-T transitions at the same sites. All occurrences of the trinucleotide sequence A-G-C, including the ochre 21 mutational hot spot, were particularly prominent sites. The selective formation of endonuclease-resistant apyrimidinic sites at specific cytosine residues may explain the high frequency of G-C to A-T transitions in the mutational spectrum of neocarzinostatin.Studies of forward mutations induced in the lacI gene of Escherichia coli by various agents have strongly suggested that mutations are targeted at sites of DNA damage (1-3). Neocarzinostatin (NCS) is an SOS-dependent mutagen that induces base substitutions and, to a lesser extent, frameshifts (4). Studies of NCS-induced mutations in the lacI gene (5) revealed that all types of monitorable base substitutions were induced, with some preference for G-C to A-T transitions (A-T to G-C transitions are not monitorable in this system). The spectrum was dominated by a G-C to A-T transition hot spot at the ochre 21 locus. Several less prominent hot spots were also present, whereas no mutations at all were detected at nearly half the monitorable sites. Since the predominant lesions induced in DNA by NCS (6, 7) or its isolated nonprotein chromophore (8) are strand breaks occurring mainly at thymine and adenine residues, it seems likely that many of the mutations, particularly those occurring at G-C base pairs, result from other, minor lesions, which are formed with a different base specificity.Since NCS releases free bases from DNA (9, 10), it is possible that apurinic/apyrimidinic (AP) sites with an intact sugar-phosphate backbone are for...

Role of exonuclease III and endonuclease IV in repair of pyrimidine dimers initiated by bacteriophage T4 pyrimidine dimer-DNA glycosylase

“…Exonuclease III seems to play a major role in the repair of typical AP sites (6), while in this study it appears that exonuclease III and endonuclease IV are equally efficient. It is possible that endonuclease IV cleaves more efficiently at the atypical AP site created by the action of T4 PD-DNA glycosylase than does exonuclease III, a result also obtained for other atypical AP sites created by bleomycin and neocarzinostatin (23,24). Laspia and Wallace have shown that exonuclease III and endonuclease IV are equally efficient in the repair of thymine glycol residues, which are initially recognized by endonuclease III (16).…”

GC/MS Methods To Quantify the 2-Deoxypentos-4-ulose and 3′-Phosphoglycolate Pathways of 4′ Oxidation of 2-Deoxyribose in DNA: Application to DNA Damage Produced by γ Radiation and Bleomycin

“…For example, base propenals react with dG to form the mutagenic pyrimidopurinone adduct, M 1 dG (9,11,17), while glyoxal adducts of dG can arise from reactions with 3′-phosphoglycolaldehyde residues generated by 3′-oxidation of 2-deoxyribose in DNA (7) and bicyclic adducts of dC, dG and dA are generated in reactions with the 2-phosphoryl-1,4-dioxobutane, products of 5′-oxidation of 2-deoxyribose (8,10,(18)(19)(20). Finally, the repair of the various base lesions and terminal sugar fragments derived from 2-deoxyribose oxidation is hindered when the lesions are present in bistranded or complex DNA lesions, such as those produced by ionizing radiation (4,21) and by enediyne and bleomycin DNA cleaving antibiotics (22,23).…”

Suppressible base substitution mutations induced by angelicin (isopsoralen) in the Escherichia coli lacI gene: implications for the mechanism of SOS mutagenesis

“…Other interpretations invoking more complicated rules for base incorporation opposite psoralen-damaged pyrimidines cannot, of course, be excluded. The preferential incorporation of pyrimidines opposite psoralen lesions would appear to be a property associated with the psoralen lesion as two other pyrimidine-damaging treatments, UV and neocarzinostatin, preferentially induce transition mutations (3,12,33).…”