Gamma-ray-irradiated medium is found to produce an immediate slowing of respiration in cells of Escherichia coli 15 <tex-math>${\rm T}^{-}{\rm L}^{-}$</tex-math>. It also introduces a lag in cell division that can be shortened by the presence of methylene blue and potassium cyanide. The lag is absent if the cells are grown anaerobically. These results suggest that the peroxides produced by irradiating medium block the electron transport system. Since cells concentrate peroxides, the loss of this function induced by bombardment was used to deduce the target molecular weight of the enzyme responsible. It is 73,000.

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