SOS-dependent A -> G transitions induced by hydroxyl radical generating system hypoxanthine xanthine oxidase/Fe3+/EDTA are accompanied by the increase of Fapy-adenine content in M13 mp18 phage DNA

Gas chromatography/isotope dilution-mass spectrometry with selected ion mon itoring (GC/IDMS-SIM) was used to measure oxidised bases in hypoxanthine/xa nthine oxidase/Fe3+/EDTA modified ss M13 mp18 phage DNA. A dose-dependent i ncrease of oxidised bases content in DNA was observed with the biggest augm entation of FapyGua, thymine glycol and FapyAde, The amount of 8-OH-Gua was relatively high both in non-oxidised and oxidised DNA, and increased to th e same extent as FapyAde and ThyGly. DNA oxidation caused a dramatic decrea se in phage survival after transfection to E, coli. Survival was improved 2 .8-fold after induction of the SOS system by UV irradiation of bacteria and mutation frequency of the lacZ gene in SOS conditions increased 7-fold ove r that in non-irradiated bacteria. Spectrum of mutations was different from those reported previously and mutations were distributed rather randomly w ithin M13 lacZ sequence, which was in contrast to previous findings, where with non-chelated metal ions other types of mutations were found in several clusters. Thus, conditions of DNA oxidation and accessibility of metal ion s for DNA bases might be important factors for generating different DNA dam ages and mutations. Major base substitutions found both in SOS-induced and non-induced E, coli but with higher mutation frequency in SOS-induced cells were C --> A (similar to 20-fold increase in SOS-conditions), G --> A (9-f old increase) and G --> C (4.5-fold increase). Very few G --> T transitions were found. A particularly large group of A --> G transitions appeared onl y in SOS-induced bacteria and was accompanied by augmentation of FapyAde co ntent in the phage DNA with undetectable 2-OH-Ade. It is then possible that imidazole ring-opened adenine mimics guanine during DNA replication and pa irs with cytosine yielding A --> G transitions in SOS-induced bacteria. (C) 1999 Elsevier Science B.V. All rights reserved.