Mutants of Salmonella typhimurium deficient in DNA polymerase. I. Detection by their failure to produce colicin E1.

Abstract

A colicinogenic strain of Salmonella typhimurium was treated with nitrosoguanidine, and the survivors were tested for spontaneous production of colicin E1. Among about 10000 clones tested, two were found which appeared to have lost the ColE1 factor and had become sensitive to methyl methanesulphonate (MMS). These two isolates also proved to be more sensitive to ultraviolet (UV) irradiation and ionizing (γ) radiation than their parent strain, and to be at least partly deficient in ability to host-cell reactivate bacteriophages damaged by UV-irradiation, γ-irradiation or MMS treatment. A third mutant with these properties has previously been described. Revertants of all three mutants selected on the basis of resistance to MMS were found to have regained wild-type resistance to UV, γ, or MMS treatment, suggesting that each of the original mutants carries a single mutation responsible for increased radiation sensitivity and reduced HCR capacity. All three mutants were of approximately normal fertility in transduction, and released temperate phages spontaneously at a significantly higher frequency than did their parent strain. Assays performed on crude extracts obtained by ultrasonic treatment established that the various mutants were deficient in an enzyme with DNA polymerase activity, and that their MMS-resistant derivates had regained almost 100% of the enzyme activity found in extracts of the wild-type parent strain. Preliminary mapping by conjugation indicated that the mutation conferring radiation sensitivity in one of the three strains lies between cysI and rha on the S. typhimurium chromosome, but attempts to determine its location more precisely by P22-mediated transduction were unsuccessful.