On the nature of the lethal and mutagenic action of ultraviolet light on frozen bacteria

The supersensitivity of bacteria to ultraviolet light (u. v. ) when irradiated at low temperatures in frozen suspension has been studied in relation to known cellular mechanisms for repair of radiation damage. Strains of Escherichia coli deficient in such mechanisms, besides being more sensitive to the lethal action of u. v., showed little or no additional supersensitivity at — 79 °C. The deficiencies studied were in (a) her locus controlling ability to excise pyrimidine dimers from DNA; (b) fil locus, probably acting at least in part by permitting more effective use of the excision-repair system; (c) rec locus, controlling an aspect of the ability to undergo recombination, presumably by action on DNA , also conferring X -ray resistance; (d) exr locus, controlling ability to repair X -ray damage, possibly by rejoining single-strand breaks. It is concluded that damage induced by u. v. at — 79 °C differs from that induced at 22 °C in being much less amenable to repair by the systems acting on DNA than damage induced in the non-frozen state. Supersensitivity of the reactivation systems themselves at — 79 °C was excluded (unless the reactivation system and the DNA are both inactivated by the same absorption event). Mutational damage induced by u. v. at — 79 °C differed from that induced at 22 °C in that the observed mutation frequency was less dependent upon immediate post-irradiation protein synthesis. Furthermore, such damage was not involved in the cumulative (‘dose-squared’) response when bacteria were thawed and irradiated further at 22 °C. The observation of Smith (see references) that the cross-linking ofDNA with protein is greater when exposure to u. v. is carried out at — 79 °C than at room temperature has been confirmed.