Treatment of Escherichia coli B/r and <tex-math>${\rm B}_{{\rm s}-1}$</tex-math> with parachloromercuribenzoate (PCMB) causes marked increases (a maximum of about 16- to 20-fold) in the gamma radiation-induced single-strand breaks in DNA with the attainment of the same high limiting yield (of the order of <tex-math>$3\times 10^{15}$</tex-math> breaks/gDNA/krad) as that reported previously for E. coli <tex-math>$15{\rm T}^{-}$</tex-math>. The amount of breaks in cells pretreated with <tex-math>$5\times 10^{-4}\ M$</tex-math> PCMB and subsequently washed five times prior to irradiation is comparable to that obtained with cells irradiated in <tex-math>$5\times 10^{-5}\ M$</tex-math> thus suggesting sensitization to involve largely cell-bound PCMB. However, no complex formation between PCMB and calf thymus DNA is detectable spectrophotometrically. Sensitization of breaks is observable at concentrations above <tex-math>$6.3\times 10^{-6}\ M$</tex-math>, a concentration which is only minimally toxic to the cells, but sufficient to inhibit postirradiation DNA degradation and to produce the same sensitivity survival-wise in both strains. In contrast, no sensitizing effect in phage λ is observed. Possible mechanisms of action of PCMB are discussed.

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