Mutagenesis vol. 13 no. 6 pp. 589-594, 1998
© 1998 UK Environmental Mutagen Society/Oxford University Press
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Formation of 8-oxoguanine in cellular DNA of Escherichia coli strains defective in different antioxidant defences
Departamento de Bioquímica y Biología Molecular, Universidad de Córdoba, Avenida de Medina Azahara s/n 14071-Córdoba, España
This paper examines the relationship in Escherichia coli between the in vivo content of 8-oxoguanine (8-oxoG) in chromosomal DNA and deficiencies of various key antioxidant defences. The structural genes for catalases (katG and katE), cytosolic superoxide dismutases (sodA and sodB) or formamidopyrimidine-DNA glycosylase (fpg) were inactivated to obtain bacterial strains lacking the scavenger enzymes for H2O2 or O2·– or the DNA repair protein for 8-oxoG. Wild-type bacteria showed 5-fold increased sensitivity to both lethality and mutagenesis by H2O2 in K medium (1 % casamino acids and 1 % glucose), as compared with nutrient broth. This higher sensitivity was associated with increased chromosomal oxidative damage, estimated as the 8-oxodG content, and with a marked decrease in both catalase and SOD activities. Bacteria lacking both cytosolic SODs (sodA sodB mutant) displayed increased 8-oxodG content in chromosomal DNA (2.8-fold that of the wild-type) when grown under standard aerated conditions. Comparatively, no significant difference in 8-oxodG content was observed in cells grown without aeration. Bacteria totally devoid of catalase activity (katG katE mutant) showed wild-type contents of 8-oxodG in chromosomal DNA when grown under aerated conditions. Nevertheless, the protective role of catalase in preventing formation of 8-oxodG in chromosomal DNA became evident under oxidative stress conditions: growth under hyperoxygenation and, particularly, following H2O2 exposure. Catalase deficiency resulted in a dramatic decrease in viability after H2O2 exposure. A deficiency of Fpg protein also sensitized E.coli to H2O2 lethality, though to lesser extent than a deficiency of catalase activity. However, the scavenger enzyme and the DNA repair protein protected equally against 8-oxoG formed in vivo upon H2O2 treatment.
1To whom correspondence should be addressed. Tel: 57 218695; Fax: 57 218688; Email: bblpucuc{at}uco.es
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