Effects of Growth Inhibition on the Ability of Escherichia coli to Survive UV Irradiation

Abstract

Escherichia coli ( E. coli ) up-regulates two major kinds of DNA repair systems to enhance their survival upon UV radiation: the error free system and the error prone system. The error free system consists of genes that code for high fidelity DNA polymerase (PolIII, PolI) and specific DNA-repair enzymes such as MutH and MutS. The error prone system, characterized by the activation of the SOS regulon, is up-regulated in stationary phase cells even in the absence of DNA damage from UV radiation. Stationary phase cells also have elevated ppGpp levels typically associated with the stringent response. One of the genes that the stringent response up-regulates is dps. The dps gene encodes for the Dps protein which functions to confer resistance to oxidative UV damage. Previous studies demonstrated that stationary phase cells are more resistance to UV killing than log phase cells (Abedi-Moghaddam, N., Bulic, A., Herderson, L., Lam, E. 2004. J. Exp. Microbiol. Immunol. 5:44-49). This phenomenon might have arisen because the cells in stationary phase were no longer replicating their DNA, and therefore avoided accumulation of mutations caused by UV treatment. In this study, culture growth was inhibited with chloramphenicol and valine to determine whether these non-growing cultures would have similar survival percentages compared to cells in stationary phase, upon UV treatment. The results show that stationary phase cells had the highest survival percentage, followed by log phase cells, and then antibiotic-treated cells. An explanation for these observations is that stringent response activation is low in log phase and completely absent in antibiotictreated cells.