It is not possible to correlate the survival of Escherichia coli strains${\rm B}/_{{\rm r}}$ and${\rm B}_{{\rm S}\text{-}1}$ with the general inhibition in the whole population of the rate of thymidine uptake soon after irradiation. At the same level of survival the more sensitive mutant${\rm B}_{{\rm S}\text{-}1}$ maintains for a short time a higher rate of thymidine uptake and DNA synthesis than does the more resistant mutant${\rm B}/_{{\rm r}}$; this is particularly noticeable after ionizing radiation in anoxic conditions. It is possible that the early relatively high rate of DNA synthesis in E. coli${\rm B}_{{\rm S}\text{-}1}$ results in the distribution of incorrect information from its radiation damaged DNA and thus a greater chance of cell death. After irradiation under aerated conditions, the rate of recovery of incorporation of thymidine into DNA is greater for E. coli${\rm B}/_{{\rm r}}$ than for E. coli${\rm B}_{{\rm S}\text{-}1}$. After irradiation under anoxic conditions, inhibition of incorporation is more marked for E. coli${\rm B}_{{\rm S}\text{-}1}$ than for E. coli${\rm B}/_{{\rm r}}$. These differences can be qualitatively related to the difference in radiation response of these two strains.

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