The range of action of genes controlling radiation sensitivity in Escherichia coli

Abstract

The range of action of the genes hcr, dar 1, dar 2, dar 3, dar 4, dar 5, and dar 6, which occur in various radiation-sensitive mutants of Escherichia coli, has been studied. From a comparative examination of these mutants and their corresponding wild types it was deduced that all the genes must be involved in the repair of lethal UV damage in DNA. The extremely sensitive mutants hcr −, dar 1 −, dar 5 − and dar 6 − showed, in contrast to the wild type, neither host-cell reactivation nor UV reactivation of phage λ. The moderately sensitive mutants dar 2 − and dar 4 − showed appreciable host-cell reactivation but no UV reactivation. The moderately sensitive mutant dar 3 −, on the contrary, showed UV reactivation but no host-cell reactivation. UV irradiation of the wild type and the mutants reduced their capacity to propagate non-irradiated phage λ to the same extent. The mutants were also indistinguishable from the wild type in their ability to propagate various irradiated RNA phages; UV irradiation reduced the capacity of the strains to propagate irradiated and non-irradiated RNA phages to the same extent. All mutants as well as the wild type were equally susceptible to thymineless death and to induction of prophage λ by thymine starvation. Ultraviolet induction of prophage λ however was achieved in sensitive mutants at a lower UV dose than in the wild type; in addition, the vegetative multiplication of the prophage after induction was more radiation sensitive in the mutants than in the wild type. All the mutants were mutually indistinguishable from each other in their prophage induction pattern. The radiation sensitivity of F′ particles in the various strains was studied. The F′ particles were far more radiation resistant than the survival of the cells themselves, in the wild type as well as in the sensitive mutants, and appeared to be subject to repair in the wild type. Irradiation of the F′ strains reduced their ability to transfer the F′ factor to female strains rather strongly; this transfer ability was more radiation sensitive in any of the mutants than in the wild-type strain, and no great differences were observed among the various sensitive mutants. From transduction experiments with an irradiated λ dg lysate it was concluded that the process leading to the formation of gal + transductants were only slightly influenced by the presence of the hcr and dar 1 genes.