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Ultraviolet- and X-Ray-Induced Responses of a Deoxyribonucleic Acid Polymerase-Deficient Mutant of Escherichia coli

The University of Tennessee-Oak Ridge Graduate School of Biomedical Sciences, and Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

ABSTRACT

Escherichia coli K-12, polAl^– is a mutant strain whose extracts are deficient in Kornberg deoxyribonucleic acid (DNA) polymerase activity. We have compared the mutant and parental strains on the basis of a number of responses to ultraviolet (UV) and X-irradiation. For both types of radiation, the mutant is more sensitive by approximately the same factor as measured by reduction in colony formation, depression of DNA synthesis, and enhancement of DNA degradation. The rate of repair of X-ray-induced single-strand breaks in the mutant is also slower, as is the repair of breaks after excision repair of UV damage. On the other hand, the mutant has a significant capability to reactivate UV-irradiated phage, although it is almost totally deficient in the ability to carry out UV reactivation. The data indicate that the polAl mutation leaves the cells with some ability to perform excision and strand-rejoining repair but that an exonuclease, whose identity remains obscure, is the agent responsible for the extensive breakdown of the DNA in polAl^– cells after irradiation.

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