Sensitization ofEscherichia ColiB/r to X-irradiation by 2′-chloro-2′-deoxythymidine

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Abstract
The sensitizing effect of 2''Cl-TdR [2''-chloro-2''-deoxythymidine] was investigated from the viewpoint of the formation of free radicals at the C-2'' position of 2''Cl-TdR when reacted with hydrated electrons. E. coli B/r cells were incubated in growth medium containing 2''Cl-TdR. Centrifugation experiments using CsCl equilibrium density gradients were carried out to confirm the incorporation of 2''Cl-TdR into DNA of the cells. The sedimentation profiles of DNA from cells grown in a medium containing 2''Cl-TdR were found at relatively heavier density positions in comparison with those of DNA from control cells. Thus, 2''Cl-TdR was incorporated into DNA. E. coli B/r cells which incorporated 2''Cl-TdR were more sensitive than the control cells for killing by X-irradiation under aerobic conditions. No difference in UV-inactivation curves between the cells grown in growth medium containing 2''Cl-TdR and the control cells was observed. From experiments using alkaline sucrose density gradient centrifugation, an increase in the frequency of radiation-induced single strand breaks of E. coli DNA containing 2''Cl-TdR was observed, compared with those of DNA from control cells. Thus, the sensitization of 2''Cl-TdR originates from an increase of damage at the sugar moiety in DNA.