Variations in the activity of glutathione reductase and the cellular glutathione content in relation to sensitivity to methylviologen in Escherichia coli

Abstract

To study the function of glutathione reductase and glutathione in Escherichia coli the coding sequence of the bacterial glutathione reductase gene ( gor gene) was cloned into the vector pBR322, and the gor gene was expressed under the control of the promoter of the tetracycline-resistance gene ( tet gene) in different Escherichia coli strains. Cells of the gor-mutant strain SG5 containing the vector pBR322 (SG5:pBR322) had no detectable glutathione reductase activity and a significantly lower total glutathione (GSH + GSSG) content relative to control cells of the strain JM101 (JM101: pBR322). The gor mutant cells were less sensitive to inhibition by methylviologen (as defined by changes in growth) than cells of the strain JM101. Elevated levels of both glutathione reductase activity and the total glutathione content (GSH + GSSG) were found when the gor gene was expressed in cells of the gor-mutant strain SG5 (SG5:pJIK1). Thus the activity of glutathione reductase is essential in order to maintain a high glutathione content. Furthermore, cells of the strain SG5: pJIK1 showed an increased sensitivity to methylviologen compared to cells of the gor mutant containing the vector pBR322 alone without the cloned gor gene insert (SG5:pBR322). In all experiments, the glutathione pool (GSH + GSSG) of bacterial cells was 90% reduced. In methylviologen-sensitive sodB mutant cells lacking iron superoxide dismutase activity (QC773:pBR322) overexpression of the cloned gor gene resulted in an elevated level of glutathione reductase activity which partially protected sodB mutant cells (QC773:pJIK1) against methylviologen toxicity. In sodB mutant cells expressing the gor gene (QC773:pJIK1) protection by glutathione reductase was, however, less effective than protection provided by expression of the iron superoxide dismutase gene ( sodB gene) in these mutant cells (QC773:pJIK2). In sodA mutant cells lacking manganese superoxide dismutase activity but expressing the cloned gor gene (QC772:pJIK1) increased cellular glutathione reductase activity did not provide protection against methylviologen.