Photoreversal of UV-potentiated glutamine tRNA suppressor mutations in excision proficient Escherichia coli

Abstract

UV-irradiated excision proficient Escherichia coli were exposed to light for photoenzymatic reversal (PR) of cyclobutane pyrimidine dimers (CPD) and assayed for reversion mutation (glutamine tRNA suppressor mutations) on semi-enriched medium or on the same medium containing acriflavine to inhibit excision repair. The initial mutation frequency without PR was relatively greater when assayed with acriflavine, and this difference increased as larger UV fluences were used. The PR kinetics were first order and about the same or slightly faster when cells were assayed with acriflavine (after 15, 30 or 45 J/m2, respectively). The results indicated mutation targeting by CPD in excision proficient cells. These results and conclusion contrast sharply with the original study of this type done several years ago. PR kinetics were considerably slower with assays containing acriflavine, sustaining the idea that PR causes repair of non-dimer targeting lesions by enhancing excision repair. To explain this contrast we devised a fluence-decrement rate for estimating the effectiveness of PR and measured PR-dependent excision repair (PER) as the difference in the fluence-decrement rate with excision proficient and deficient cells. PER was more evident when cells were prepared as in the original study but was still an insufficient factor. More importantly, the original study included a component of indirect photoreactivation or photoprotection (using unfiltered PR light) which accentuated the role of excision repair. Taking these factors into account, the original data also are consistent with the model that glutamine tRNA suppressor mutations produced by UV-mutagenesis in excision proficient E. coli result from targeting by CPD just as in excision defective cells. Thus, with regards to a common UV mutation assay, there does not appear to be two types of targeting lesion depending on excision proficiency.