X-Ray-Induced Degradation of Deoxyribonucleic Acid in a Polyauxotrophic Bacterium

Abstract

It has been shown that exposure of bacteria to X-rays results in the loss of deoxyribonucleic acid (DNA) during postirradiation growth (1, 2). Furthermore, irradiated bacteria incubated under starvation conditions (3) or in the presence of chloramphenicol (4) exhibit an increase in both rate and extent of DNA loss. Inhibition of protein and ribonucleic acid (RNA) synthesis by a period of amino acid starvation before irradiation has been reported to increase the resistance of bacteria to the killing effects of X-rays (5). This acquired resistance to X-ray killing, however, is reversed if protein and RNA synthesis are allowed to resume before irradiation (5). The concept currently held is that in the absence of protein and RNA synthesis bacteria complete chromosome replication but are unable to initiate another cycle of chromosome synthesis unless a period of protein and RNA synthesis precedes the return to a thymine-supplemented medium (6, 7). It is not unreasonable to assume that certain stages in the process of chromosome replication may be either more resistant or more sensitive to the effects of radiation and could contribute to variations in radiation sensitivity. In addition, the physiological state of the cell at the time of exposure could influence the response of the damaged replicative system to postirradiation repair. In this investigation, a comparison was made between (1) postirradiation DNA degradation in bacteria removed from cultures in logarithmic growth and (2) postirradiation DNA degradation in bacteria that had completed chromosome replication and had ceased active DNA synthesis. Observations reported here concern the effects of X-rays on the stability and characteristics of DNA in bacteria exposed to various conditions of preirradiation and postirradiation nutritional supplementation.