Paraquat generates superoxide inside the cell by NADPH-mediated reduction of oxygen. Superoxide is sensed by SoxR, which enhances the expression of the transcription factor SoxS, which in turn stimulates other downstream targets. One of the SoxS-regulated genes, sodA (superoxide dismutase), converts superoxide into hydrogen peroxide, which acts as a signal to enhance transcription of genes through OxyR. katG (catalase), a hydrogen peroxide scavenger is an OxyR-regulated gene. Superoxide and hydrogen peroxide also directly affect IscR modulation of a set of genes involved in iron sulfur cluster biogenesis.