Increased sensitivity to sparsely ionizing radiation due to excessive base excision in clustered DNA damage sites in Escherichia coli

Abstract

Purpose: In order to clarify the cellular processing and repair mechanisms for radiation-induced clustered DNA damage, we examined the correlation between the levels of DNA glycosylases and the sensitivity to ionizing radiation in Escherichia coli.

Materials and methods: The lethal effects of γ-rays, X-rays, α-particles and H₂O₂ were determined in E. coli with different levels of DNA glycosylases. The formation of double-strand breaks by post-irradiation treatment with DNA glycosylase was assayed with γ-irradiated plasmid DNA in vitro.

Results: An E. coli mutM nth nei triple mutant was less sensitive to the lethal effect of sparsely ionizing radiation (γ-rays and X-rays) than the wild-type strain. Overproduction of MutM (8-oxoguanine-DNA glycosylase), Nth (endonuclease III) and Nei (endonulease VIII) increased the sensitivity to γ-rays, whereas it did not affect the sensitivity to α-particles. Increased sensitivity to γ-rays also occurred in E. coli overproducing human 8-oxoguanine-DNA glycosylase (hOgg1). Treatment of γ-irradiated plasmid DNA with purified MutM converted the covalently closed circular to the linear form of the DNA. On the other hand, overproduction of MutM conferred resistance to H₂O₂ on the E. coli mutM nth nei mutant.

Conclusions: The levels of DNA glycosylases affect the sensitivity of E. coli to γ-rays and X-rays. Excessive excision by DNA glycosylases converts nearly opposite base damage in clustered DNA damage to double-strand breaks, which are potentially lethal.

• Clustered DNA damage
• DNA glycosylase
• E. coli
• double-strand breaks
• radiosensitivity