Superoxide dismutase which is metalloenzyme that decomposes superoxide radicals into hydrogen peroxide and molecular oxygen. Vitreoscilla has FeSOD. Expression of FeSOD to paraquat was largely constitutive. This suggests that the basal level of FeSOD is sufficient to provide protection against superoxide generated during normal aerobic metabolism. Induction of SOD by iron supports that insertion of the active site metal into the corresponding apoprotein. The effect of paraquat on induction by iron seemed that iron brought the synergism effect in SOD activity with paraquat. It suggests that the relief of growth inhibition is due to protection against the lethality of O$_2$afforded by the elevated SOD. There may be control of FeSOD activity posttranslationally. Posttranslation control of enzyme function is particularly feasible for a metalloenzyme, for which conversion of apo- to holoenzyme may be the rate-limiting or regulatory step.