Mutation spectra analysis suggests that N-(2-chloroethyl)-N′-cyclohexyl-N-nitrosourea-induced lesions are subject to transcription-coupled repair in Escherichia coli
✍ Scribed by Raffaella Iannone; Alberto Inga; Francisco L. Luque-Romero; Paola Menichini; Angelo Abbondandolo; Nieves Abril; Carmen Pueyo; Gilberto Fronza
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 115 KB
- Volume
- 19
- Category
- Article
- ISSN
- 0899-1987
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✦ Synopsis
To determine the influence of some bacterial DNA repair pathways on the mutagenic and the lethal effects of N-(2-chloroethyl)-N´-cyclohexyl-N-nitrosourea (CCNU), pZ189 plasmids treated in vitro with 2 mM CCNU were transfected into Escherichia coli strains with different repair capacities (uvr + ada + ogt + , uvr -ada + ogt + , and uvr -ada -ogt -). Despite the differences in repair capacities, no statistically significant difference in survival and mutability was observed among the tested strains. One hundred and sixty-six CCNU-induced supF mutants were isolated and sequenced. All mutants were characterized by single base-pair substitutions, most of which (more than 96%) were GC→AT transitions (the mutated G being almost exclusively preceded 5´ by a purine). Mutation distribution was not random. Position 160 (5´-GGT-3´, nontranscribed (NT) strand) was a uvr + ada + ogt +specific hot-spot. Position 123 (5´-GGG-3´, NT strand) was a common hot-spot but significantly more mutable in repair-proficient strains than in repair-deficient strains. Conversely, position 168 (5´-GGA-3´, transcribed (T) strand) was significantly more mutable in repair-deficient strains than in repair-proficient strains. By applying a computer program for comparison of mutational spectra, we found that the uvr + mutational spectrum was significantly different from those obtained in uvr -strains, whereas in the uvr -background, no difference was observed between mutation spectra in ada + ogt + versus ada -ogt -strains. Our results are consistent with the hypothesis that O 6 -alkylguanine is responsible for most mutations observed in all strains. The results also indicate that excision repair modulates the distribution of GC→AT transitions. The fact that mutations at G lesions on the T strand were significantly less frequent in uvr + than in uvr -strains suggests that CCNU-induced premutational lesions are susceptible to strand-preferential repair in E. coli.