ANTIOXIDANT VITAMIN-C AND VITAMIN-E AFFECT THE SUPEROXIDE-MEDIATED INDUCTION OF THE SOXRS REGULON OF ESCHERICHIA-COLI

The mechanism of activation of Escherichia coil redox sensory protein SoxR is still unclear: a [2Fe-2S] cluster contained in a SoxR dimer is potentially redox-sensitive, but the nature of the signal is unknown. Antioxidant vitamins C (ascorbate) and E (alpha-tocopherol) were used to explore the mechanism of activation of the SoxR protein in vivo. T reating E. coil cells with ascorbate or cr-tocopherol increased their tolerance to paraquat (PQ, a redox-cycling compound), even in the abse nce of the soxRS locus, suggesting a radical-quenching activity. When using a soxS'::lacZ fusion, whose expression is governed by activated SoxR, ascorbate and alpha-tocopherol also prevented the expression of beta-galactosidase after PQ treatment. A secondary activity was observ ed in cells carrying soxR101. a mutation resulting in the constitutive expression of the sox regulon, where the overexpression of soxS':: la cZ was also reduced by ascorbate or alpha-tocopherol treatment. Additi onally, different mechanisms of action were revealed as alpha-tocopher ol was capable of preventing both PQ and menadione (MD) lethality, whi lst ascorbate prevented PQ lethality but increased MD-mediated cell de ath. It is proposed that alpha-tocopherol. positioned in membranes, ca n prevent superoxide-dependent membrane damage; however, water-soluble ascorbate is unable to do so and can even increase the concentration of oxygen radicals reading with released membrane-associated Fe(ll).