The Repair of Single-Strand Breaks in a Radiosensitive Mutant of Micrococcus radiodurans

Abstract

The kinetics of x-ray-induced single-strand break rejoinirg have been studied in wild-type M. radiodurans and a radiosensitive mutant, UV17. The initial fast rejoining reaction observed in wild-type cells in growth medium was absent in UV17. Studies in which irradiated cells were allowed to repair under various conditions confirm that a buffer-mediated rejoining system is absent in UV17 while rejoining of strand breaks under growth conditions is slower but not absent DNA breakdown is more extensive in UV17 although its rate is similar to that of the wild-type. In both strains the x-ray-induced DNA synthesis delay and the time for maximal release of acid-soluble material correlate. In vitro studies show the capacity of lysates of wild-type and mutant cells to catalyze the incorporation of deoxyribonucleotide triphosphates into acid-insoluble material are similar while DNase activity in the mutant is much greater than in the wild type. The in vitro data are correlated with the in vivo observations and indicate that the sensitivity of UV17 to ionizing radiation may be the result of an inhibition of DNA polymerase-mediated repair and not a consequence of an exr or lex type of mutation as has been previously thought.