Absence of Pyrimidine Salvage and Prevention of Thymineless Radiosensitization in Escherichia coli thyA Cells Fed Dihydrothymine or Thymine Glycol

Little information is available concerning the metabolic fate of radiation-induced thymine base damage products once they have been excised from DNA. The present study was an attempt to determine whether or not thymine-requiring mutants of Escherichia coli could grow on dihydrothymine (DHT) and thymine glycol (TG) by "salvaging" the altered thymines. A second test of thymine product utilization was prevention of thymineless radiosensitization. Results showed that very low growth of ${\rm Thy}^{-}$ cells on DHT or TG could be explained by the presence of ≤1% contaminating thymine in the mixtures. Radiation dose-modification factors (DMFs) for thyA cells fed DHT or TG for 3 h were 1.38 ± 0.28 and 1.26 ± 0.24, respectively, whereas the DMF for 3 h thymine-starved cells was 1.63 ± 0.05. The small (∼25%) amelioration of thymineless radiosensitization observed in DHT- or TG-fed cells could probably be explained by contaminating thymine in the medium. Although DHT is a normal metabolite in some cells, neither DHT nor TG could be used efficiently by thymine-requiring cells in the protocol presented.