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4-nitroquinoline-1-oxide (NQO) induces high frequencies of intragenic revertants of amber (UAG) but not ochre (UAA) mutants of yeast. Distinction of the amber and ochre codons was made with well-characterized nonsense mutants of the iso-1-cytochromc c gene (cycl mutants) as well as with nonsense mutants having nutritional requirements. Thus the NQO-induced reversion frequencies corroborated the assignments that wece based on the pattern of amino acid replacements in intragenic revertants and on the speficity of suppression. It was concluded from these results and from the results of a previous investigation with other cycl mutants (PRAKASH, STEWART and SHERMAN 1974) that NQO induces transversions of G:C base pairs at many sites and that the specificity is not strongly influenced by neighboring base pairs in at least the strains examined in these studies. NQO was previously shown to induce G: C → A: T transitions at least at one site and this and the previous study established that it does not significantly mutate A:T base pairs at numerous sites. Thus NQO can be used to selectively mutate G:C base pairs and to determine if the pathways of reverse mutations involve G:C base pairs. Suppressors that act on either amber or ochre mutants were induced with NQO, indicating that they can arise by mutations of G:C base pairs.

DIFFERENTIATION BETWEEN AMBER AND OCHRE MUTANTS OF YEAST BY REVERSION WITH 4-NITROQUINOLINE-1-OXIDE