Autoradiographic detection of mutation to exotoxin-A resistance in mouse fibroblasts treated with ethyl methanesulfonate, X-rays and ultraviolet light

Variable aberrant cDNAs in single diphtheria toxin-resistant human fibroblasts

We treated transformed human fibroblasts with diphtheria toxin (DT) and isolated 40 single cells that were toxin resistant but unable to propagate. In 13 of them toxin resistance was associated with the presence of one or more aberrant transcripts of the structural gene for elongation factor 2 (EF-2). cDNA obtained from these transcripts had 164-447 bp-long deletions. Each of these deletions was associated with 2-8 base pairs-long repeats at its breakpoints. Only 10 out of 16 cDNA deletions were associated with presumed exon junctions. A role is suggested for errors in transcription in producing the aberrant transcripts which gave rise to the deletion-bearing cDNA species.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 2. Spontaneous mutations in mammalian cells

Spontaneous or background mutation in mammals plays an important role in both medical and evolutionary contexts. However, establishing mutation frequencies or rates has not always been easy. When the field of mammalian mutagenesis was in its infancy, the word "variant" rather than "mutant" was often used because the genetic nature of the observed phenotypic alterations could not be adequately proven. Nowadays numerous target genes have been identified in which mutant frequencies can be measured, and occasionally even rates can be estimated. Indeed, the genetic basis for 'variants' now often comes from direct DNA sequencing. This review describes the most often used and best understood genetic markers for mutation research and examines their usefulness. In addition, mutational specificity is compared for several loci and the use of DNA-sequence data in determining the origins of spontaneous mutation is also discussed. An important observation is that spontaneous mutation frequencies of similarly sized genes can vary by more than an order of magnitude. Chromosomal location, the nature of the gene product and mutational specificity may offer a partial explanation.

Dominant lethal cell mutants detected by the autoradiographic assay for exotoxin A resistance

The autoradiographic assay (AR assay) for P. aeruginosa exotoxin A (PE) resistance in cultured mouse fibroblasts detects mutants able to synthesize proteins in the presence of the toxin, presumably due to mutations in the structural gene for elongation factor 2 (EF-2). Detection by the AR assay of PER cells is independent of their ability to divide. The frequencies of both spontaneous and mutagen-induced PER cells are higher than those detected by the conventional colony assay. Examination of phenotypic expression times in the PER cells, and of their in situ proliferation, reveals that this higher sensitivity of the AR assay is due to its ability to detect cells in which the PER mutation prevents proliferation, thus escaping detection by the colony assay. Expression of the mutant phenotype in the PER cells detected in the AR assay after mutagenesis with ethyl methanesulfonate (EMS) follows a pattern similar to that observed in the colony assay, reaching a maximum in 3 days, and then remaining constant for at least 4 more. After treatment with X rays (which fail to induce PER mutants in the colony assay), the frequency of PER cells detected in the AR assay also reaches a maximum on day 3, but then declines sharply, returning to the spontaneous level on day 7. In the absence of PE, the majority of the spontaneous or mutagen-induced PER cells detected in the AR assay are either incapable of dividing at all, or capable of undergoing a limited number of cell divisions to produce micro-colonies. Only few of them may continue to grow into 'full-size' colonies comparable to those detected in the colony assay. In the presence of the toxin, the proportion of PER cells which are able to divide is even smaller, and that of cells able to form full-sized PER colonies detectable in the AR assay is comparable to the results obtained in the conventional colony assay. We presume that the lethality of the PER mutations in the cells detected by the AR assay is due to abnormal protein synthesis resulting from the same mutational change that made these cells resistant to PE. While incapable of supporting colony formation, and hence detection by the colony assay, such abnormal protein synthesis still allows the detection of the mutant cells by the AR assay.