Sensitivity to ionizing radiations and damage repair in yeast

Cultures of haploid and diploid clones of Saccharomyces cerevisiae are cell populations heterogeneous for their radiation sensitivity. A fraction highly resistant to ionizing radiation was recognized to be constituted by budding cells in G 2 or late DNA synthesis stage while sensitive cells are those in G 1. The large amount of data obtained on the rad mutants (sensitive to either ionizing radiations or U.V. or to both) has led to the development of complex models of repair pathways which will be discussed in some detail. Most of the work that has been done to detect any specificity of repair mechanisms on point mutations has dealt with the effect of rad mutants on the rate of induction of base substitutions by U.V. light and chemical mutagens. Very little, however, is known for X-radiation induced mutations and practically no evidence has been accumulated on the effect of any mutagen on the induction of base addition/deletion mutations. The general pattern can be easily outlined by saying that excision repair mechanisms are generally error free. Strains carrying rad mutants of the rad1 group are much more sensitive than the wild type to point mutation induction by U.V. and some chemicals. The rad6 group affects functions substantially error prone, while the functions more specific for repairing X-radiation induced damage ( rad50 group) seem to be devoid of any effect in point mutations.