UV-induced mutation and recombination in yeast: Involvement of an inducible component of repair.

Abstract

Frequency curves for UV-induced mutation and mitotic recombination are linear at low doses in many strains of S. cerevisiae. At higher doses these curves either increase at higher powers of dose and/or reach a maximum induced frequency and then decline. Previously, we have shown that such dose-response patterns can arise from: (I) non-linearities inherent in the molecular mechanism involved; and (2) "@-effects", i.e. differential probabilities of clone formation for the mutant and non-mutant cells. Non-linearities of the first sort could arise from inducible components of mutagenesis or recombinagenesis, from reductions in the efficiency of error-free repair of from the occurrence of special "two-hit" lesions. " ~-ef~ fects" are associated with expression of the mutant phenotype, or with mutant clone formation. We now have shown that the existence of higher order components of mutagenesis or recombinagenesis is easily determined by examining the ratio of the induced mutant yield to the linear component of mutant frequency as a function of dose. For purely linear responses, and no JJ ~ n -effects , a plot of this ratio (a quantity we call"apparent survival") versus dose generates the actual survival curve of the cells. If the non-linear frequency components arise solely from "@-effects", then the "apparent survival" is the survival curve for the initially induced mutant or recombinant subpopulation. If the "apparent survival" is greater than unity, then there must exist positive, non-linear components associated with the mutational or recombinational processes themselves. Such components exist in RAD wild-type cells undergoing reversion or gene conversion to prototrophy, and are sensitive to inhibition of protein synthesis by cycloheximide. Thus, they may be ascribed to inducible error-prone repair or recombination. Such components do not appear in excision-deficient mutants undergoing forward muation or mitotic crossing over. The high dose decline in frequency, which is observed for forward mutation in RAD and rad2 strains, and for gene conversion and crossing-over in RAD and rad3 diploids, can be attributed to "@-effects". (Supported by NSERC Canada)