The yield of single-strand breaks per single-strand genome measured 0.2 sec after irradiation of Escherichia coli B/r or Pol${\rm A}{}_{1}{}^{-}$ with 4.3-MeV electrons was found to be 0.9 breaks/krad in anoxia and 3.2 breaks/krad in the presence of oxygen. By following the increase in the number of breaks due to endonuclease action in Pol${\rm A}{}_{1}{}^{-}$ it was found that 1.3 endonuclease-sensitive sites were present for each radiation-induced break. By adding oxygen shortly after anoxic irradiation it was shown that (i) the presence of oxygen is required for full endonuclease activity and (ii) that oxygen is truly dose modifying with respect to the number of radiation-induced breaks and endonuclease-sensitive sites.

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