Simultaneous induction of multiple mutations by N-methyl-N'-nitro-N-nitrosoguanidine in the yeast Saccharomyces cerevisiae

Electrophoretic analysis of proteins from Phycomyces mutants with abnormal tropisms

Certain phototropism mutants of Phycomyces blakesleeanus show defective bending responses (tropisms) to stimuli besides light, such as gravity, wind, and barriers. These so-called "stiff" mutants are affected in four genes (madD to madG). Using two-dimensional gel electrophoresis, we have analyzed polypeptides from microsomal and soluble fractions obtained from the wild type, four single mutants, and six double mutants affected in all pairwise combinations of the four genes. Consistent differences in spot patterns for madE and madF mutants were found in microsomal fractions but not in soluble fractions. In madE mutants, two spots designated E1 (52 kDa, pI 6.65) and E2 (50 kDa, pI 6.65) were altered. E1 appeared denser in the wild type than in the madE mutants, while the reverse was true for E2. The spots E1 and E2 are probably under regulatory control by madE, perhaps involving posttranslational modification. A protein spot, F1 (53 kDa, pI 6.1), was present on the wild-type gels but absent from all gels for madF mutants. The F1 polypeptide probably represents the madF gene product.

Induction and isolation of mutants in fungi at low mutagen doses

Since the yield of mutants per surviving cell increases in general with increasing dose of mutagen, it has often been concluded in the literature that it is the most efficient to apply high mutagen doses so that most spores are killed. As high doses of mutagen produce chromosome rearrangements and unnoticed mutations which disturb the genetic background, the relationship between mutant frequency and survival was analyzed with Aspergillus nidulans as a model. It is shown that for different types of mutants the highest mutant yield is obtained at low mutagen doses (20-50% survival). Mutant frequency increases with increasing dose of mutagen but levels off and even decreases at higher dosages. There is no simple linear relationship between mutant frequency and the logarithm of the mutagen dose or the logarithm of the surviving fraction. If appropriate enrichment procedures are also available auxotrophic mutants can best be isolated at low doses of mutagen. Taking into account the disturbance of the genetic background, mutation induction should be done preferentially at a survival level of at least 70%.

Alkylation mutagenesis in Saccharomyces cerevisiae: lack of evidence for an adaptive response

We have found no evidence for an adaptive response for either lethality or mutagenesis following treatment of Saccharomyces cerevisiae with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The rad6 and rad52 mutants of S. cerevisiae are highly defective in MNNG and ethyl methanesulfonate induced mutagenesis of both stationary and exponential phase cells. These and other observations indicate that the mechanisms of repair of alkylation damage and mutagenesis differ markedly between S. cerevisiae and Escherichia coli.