UV-induced replicating instability in Schizosaccharomyces pombe

Replicating instabilities in yeast: occurrence in different mutational systems

Following mutagenesis of yeast cells with nitrosoguanidine, primary mosaic colonies exhibiting prototrophic/auxotrophic phenotypes were obtained. Upon replating of these primary mosaics, numerous secondary mosaics were present in the progeny. This study shows that replicating instabilities occur at many different loci within the Schizosaccharomyces pombe genome. In addition, the ade-1 gene of Saccharomyces cerevisiae (causing red pigmentation) was used to show that the phenomenon also occurs in this yeast.

On the nature and specificity of replicating instability in fission yeast

A strain of fission yeast carrying replicating instability (RI) will segregate mitotically three types of cells: unstable (still RI-carrying) cells, stable identical mutants and stable non-mutants. RI in fission yeast has previously been considered as a specific type of premutational lesion capable of (1) being replicated as such and (2) reverting at an appreciable rate to the normal state as well as changing into a stable mutation. In the present work genetic analysis of a previously studied RI-carrying strain showed this strain to be a diploid, most probably heterozygous for a recessive mutation. It was possible to construct other unstable heterozygous strains segregating predetermined mutants but not to induce RI by UV-irradiation of haploid cells. Thus evidence is presented against the premutational nature of RI in fission yeast.

Ascertainment of the effect of differential growth rates of mutants on observed mutant frequencies in X-irradiated mammalian cells. An indication for the occurrence of X-ray-induced untargeted mutagenesis

As it is not known to what extent differential growth rates of induced mutants lead to over- and under-representation of mutants in treated populations and thereby affect the determination of mutant frequencies, the mutation induction in X-irradiated L5178Y mouse lymphoma cells was determined via two methods. The first method involves the standard protocol which may suffer from the effect of differential growth rates, while the second method is based upon the fluctuation test in which the differential growth rates can be actually measured. It appeared that the standard protocol led to a mutant frequency that was similar to the mutant frequency determined in the fluctuation test. Therefore, the standard protocol appears to lead to only a minor under-estimation if any. Substantial heterogeneity in growth rates of induced mutants was observed, but the mutants with a selective advantage appear largely to compensate for the mutants that are lost because of selective disadvantage. It was calculated that the chance for isolating the same mutant twice from a treated population had been increased 2.2-fold because of the observed differential growth rates. Therefore, our data indicate that the standard protocol does not lead to serious errors in the determination of mutant frequencies and in the sampling of mutants. The fluctuation tests were also used to determine the spontaneous mutation frequency per cell per generation. The mutation rate appeared more than 10-fold enhanced in X-irradiated cells which may be attributed to the induction of a process of untargeted mutagenesis in mammalian cells.

Excision of pyrimidine dimers by several UV-sensitive mutants of S. pombe

Nine radiation-sensitive mutants of S. pombe showing a variety of phenotypic characteristics were analysed for their ability to excise pyrimidine dimers after ultraviolet irradiation. From earlier studies using indirect parameters, it was expected that some would be excision-deficient. Data reported here show that all the mutants tested, like wild type cells, were able to remove a high percentage of pyrimidine dimers during post-irradiation incubation in several different holding media, but not in saline or phosphate buffer. These mutants included strains showing increased, as well as others which showed decreased, levels of UV-induced mutation frequency relative to that of the wild type at the same total dose.