Survival and SOS Response Induction in Ultraviolet B irradiated Escherichia coli Cells Defective to DNA Repair Mechanisms

Abstract

Escherichia coli react to ultraviolet radiation by activating a cellular response against genotoxic stress known as SOS response. In this paper, the importance of different gene products involved in DNA repair to both survival and SOS activation in E. coli strains exposed to ultraviolet B radiation was evaluated. For this purpose, we determined cell survival through the plate counting method in E. coli strains carrying uvrA, oxyR, recO, recN, recJ, exoX, recB, recD, fpg and xonA mutations and in the wild type strain. These strains also contain the genetic sulA::lacZ fusion that allows the quantification of the SOS activity by mean of the SOS chromotest. Based on survival data five gene products were particularly important to cell survival as follow: UvrA > RecB > RecO > RecJ > XonA. UvrA and recJ defective strains showed an elevated SOS induction related to the wild type strain and such activity is sustained up to 240 minutes of post-irradiation incubation; indicating that sulA function is not turned off in E. coli cells with defective function for UvrA and RecJ proteins. In addition, E. coli strains carrying recO and recN mutations did not induced the SOS response after UVB- irradiation, suggesting that RecO and RecN proteins are essential to SOS activation, while RecD protein had a secondary role in this process. Here, we provide new roles to DNA repair proteins and new inside into mechanisms to activate SOS response in E. coli.