Polyamines reduce paraquat-induced soxS and its regulon expression in Escherichia coli

Polyamines, ubiquitous polycationic compounds, are involved in many cellular responses and relieve paraquat-induced cytotoxicity in Escherichia coli. We constructed a new E. coli mutant strain, JIL528, which is deficient in the biosynthesis of both putrescine and spermidine, to examine the physiological role of polyamines under oxidative stress caused by paraquat. Putrescine and spermidine downregulate the expression of soxS induced by paraquat in a concentration-dependent manner. The product of SoxS is a key regulator governing cellular responses against oxidative stress in E. coli. The downregulation of soxS expression by polyamines was not shown in the soxR mutant background. Glucose-6-phosphate dehydrogenase (G6PDH; encoded by zwf) and manganese-containing superoxide dismutase (Mn-SOD; encoded by sodA) activities induced by paraquat were decreased by exogenous polyamines. The induction of the zwf expression by paraquat was also decreased by exogenous polyamines. The polyamine-deficient mutant strain JIL528 showed a higher soxS expression than its parent polyamine-proficient wild type BW1157, on exogenous supplementation of paraquat concentrations below 1 micromol/L. While the growth rate of the mutant was decreased, soxS expression was increased in a concentration-dependent manner above 0.01 micromol/L of paraquat. In contrast, growth inhibition of the mutant by paraquat was relieved, and soxS was no longer induced by exogenous putrescine (1 mmol/L). In conclusion, polyamines protect against paraquat-induced toxicity but downregulate soxS expression, suggesting that the protective role of polyamines against oxidative damage induced by paraquat results in soxS downregulation.