Mutagenic specificity: reversion of iso-1-cytochrome c mutants of yeast

In previous studies the nucleotide sequences of numerous mutant codons in the cy1 gene have been identified from altered iso-1-cytochromes c. These studies not only revealed the mutant codons that caused the deficiencies but also experimentally determined which of the base pair changes allowed the formation of functional iso-1-cytochromes c. In this investigation we have quantitatively measured the reversion frequencies of eleven cy1 mutants which were treated with 12 mutagens. The cy1 mutants comprised nine mutants having single-base changes of the AUG initiation codon (Stewart et al., 1971), an ochre mutant cy1–9 (Stewart et al., 1972), and an amber mutant cy1–179 (Stewart &; Sherman, 1972). In some cases the types of induced base changes could be inferred unambiguously from the pattern of reversion. Selective G.C to A.T transitions were induced by ethyl methanesulfonate, diethyl sulfate, N-methyl-N′-nitro-N-nitrosoguanidine, 1-nitrosoimidazolidone-2, nitrous acid, [5-³H]uridine and β-propiolactone. There was no apparent specificity with methyl methanesulfonate, dimethyl sulfate, nitrogen mustard and γ-rays. Ultraviolet light induced high rates of reversion of the ochre and amber mutants, but in these instances it appears as if the selective action is due to particular nucleotide sequences and not due to simple types of base pair changes.