Induction of phr gene expression in E. coli strain KY706/pPL-1 by He–Ne laser (632.8 nm) irradiation

Abstract

We have observed that He–Ne laser irradiation of E. coli strain KY706/pPL-1 leads to induction of photolyase gene, phr. The magnitude of induction was found to depend on the He–Ne laser fluence, fluence rate and post-irradiation incubation period in the nutrient medium. The optimum values for fluence and fluence rate were 7×10 3 J/m 2 and 100 W/m 2, respectively, and the induction of phr gene was observed to saturate beyond an incubation period of ∼2 h. Experiments carried out with singlet oxygen quenchers and with D 2O suggest that the effect is mediated via singlet oxygen. Photoreactivation studies carried out after UVC exposure of both the He–Ne laser-exposed as well as unexposed cells showed a larger surviving fraction in the He–Ne laser pre-irradiated cells. This can be attributed to He–Ne laser irradiation-induced induction of phr expression. However, since even without photoreactivating light He–Ne laser pre-irradiated cells show higher survival against UVC radiation it appears that He–Ne laser irradiation induces both light-dependent as well as dark DNA repair processes.