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.