Abstract
Experiment has been carried out using UV-sensitive mutant of uvs 1 of Saccharomyces cerevisiae, presumably lacking ability of dark repair, to investigate genetic nature of UV-induced premutational damages leading to three-different types of mutation under the dark and illuminated conditions. Yields of these types of mutation in uvs 1 are markedly higher than those in wild type at equal doses. At the dark condition, logarithmically plotted dose-response curves of true-back mutations from arg 4-17, his 5-2 and lys 1-1 induced in uvs 1 consist of two straight lines. Slopes of the curves at high doses are more steeper than those at low doses and almost the same as those in wild type in all the dose range. The curves for induced frequency of super-suppressor mutation and that of back mutation from his 1-1, i.e., addition-deletion type mutation consists of one component, and their slopes are nearly equal for both uvs 1 and wild-type strains. Furthermore, ratios of true back mutation yields between uvs 1 and wild-type strains at equal doses are greater than those of super-suppressor mutations or reversion of addition-deletion type. To account for these results it is postulated that premutational damage leading to true-back mutation is preferentially repairable compared with damages responsible for the other types of mutation. Molecular natures of the premutational damages leading to the three types of mutation are discussed.
