Escherichia coli <tex-math>$15{\rm T}^{-}$</tex-math> cells have been irradiated by gamma rays at 21°C at a dose rate of 1.3 krad/min in the presence and absence of parachloromercuribenzoate (PCMB) and the effects of hydroxyl radical scavengers on the yields of single-strand DNA breaks under atmospheric and anoxic conditions (the latter achieved with the aid of sodium dithionite) studied. In the absence of PCMB, the amount of breaks, <tex-math>$1.9\times 10^{14}$</tex-math> breaks/g of DNA/krad, is little affected by the removal of OH radicals or oxygen, and represents the result of broken DNA that has largely been rejoined under the conditions employed. PCMB increases single-strand breaks under atmospheric and anoxic conditions by 18- and 7-fold, respectively. A significant amount of such breaks can be reduced by OH radical scavengers, and a study of the kinetics of scavenging under atmospheric conditions by four such scavengers shows that OH radicals are responsible for 79% of the breaks in PCMB-treated cells. In the absence of PCMB, the cells are capable of repairing all the breaks produced by OH radicals as well as some by the direct effect. The sensitizing effect of PCMB is attributable to its inactivation of intracellular repair enzymes. Other plausible modes of action of PCMB are also discussed.

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