A tester system for detecting each of the six base-pair substitutions in Saccharomyces cerevisiae by selecting for an essential cysteine in iso-1-cytochrome c

A collection of isogenic yeast strains that is specifically diagnostic for the six possible base-pair substitutions is described. Each strain contains a single, unique base-pair substitution at the Cys-22 codon of the CYC1 gene, which codes for iso-1-cytochrome c. These mutations encode replacements of the functionally critical Cys-22 and render each strain unable to grow on media containing nonfermentable carbon sources (Cyc^-). Specific base-pair substitutions, which restore the Cys-22 codon, can be monitored simply by scoring for reversion to the Cyc⁺ phenotype. These strains revert spontaneously at very low frequencies and exhibit specific patterns of reversion in response to different mutagens. Only true (CYC1⁺) revertants were recovered after 7 days on selection medium. The following mutagen specificities were observed: ethyl methanesulfonate and N-methyl-N'-nitro-N-nitrosoguanidine, G·C→A·T; 4-nitroquinoline-1-oxide, G·C→T·A and G·C→A·T; diepoxybutane, A·T→T·A, A·T→G·C and G·C→T·A; 5-azacytidine, G·C→C·G. Methyl methanesulfonate induced all six mutations, albeit at relatively low frequencies, with preference for A·T→T·A and A·T→G·C. Ultraviolet light was the most inefficient mutagen used in this study, consistent with its preference for transition mutations at dipyrimidine sequences reported in other systems. This tester system is valuable as a simple and reliable assay for specific mutations without DNA sequence analysis.