Dose-rate dependence of mutagenesis by ultraviolet radiation and 4-nitroquinoline 1-oxide in Escherichia coli.

Abstract

To study the dose-rate dependence of mutagenesis in Escherichia coli by ultraviolet radiation (UV) and UV-mimetic chemical, 4-nitroquinoline 1-oxide (4NQO), equal total doses of UV or 4NQO were given to bacteria at high dose- rates in single doses and at low dose-rates by continuous exposure or in fractionated doses during a 7 hr period of holding the cells in buffer. Yields of mutations to prototrophy in a repair-proficient strain greatly decreased after low dose-rate treatment with either UV or 4NQO when compared to those under high dose-rate treatment. On the other hand, a uvrA- derivative of the repair-proficient which lacks excision rapair ability, showed no reduction in UV- or 4NQO-induced mutation frequencies by lowering the dose-rate. These results support the assumption that excision repair works at higher efficiency for UV- or 4NQO-induced damage in DNA under low dose-rate treatment than under high dose-rate treatment. Mutagenicity of 4NQO was reduced about 20 times in the repair-proficient strain by lowering the dose-rate. The mutation-frequency-versus-dose curve showed, however, no indication of a threshold dose below which bacteria with proficient DNA repair become resistant to mutagenesis.