Specific induction of transitions and transversions of G · C base pairs by 4-nitroquinoline-1-oxide in iso-1-cytochrome c mutants of yeast

The base-pair changes induced by the highly carcinogenic agent, 4-nitroquinoline-1-oxide, have been determined from the reversion rates of defined tester strains and from the amino acid replacements of revertant iso-1-cytochromes c. The mutant codons and the base-pair changes of reverse mutations of 14 cyc1 mutants were previously determined from alterations of iso-1-cytochromes c in intragenic revertants. These 14 cyc1 mutants, which were used as tester strains, included nine mutants with altered AUG initiation codons, an ochre (UAA) mutant, an amber (UAG) mutant and three frameshift mutants (Stewart et al., 1971,1972; Stewart & Sherman, 1972,1974; Sherman & Stewart, 1973). NQO^† † Abbreviation used: NQO, 4-nitroquinoline-l-oxide. induced a high rate of reversion in the initiation mutant cyc1-131, the only mutant in the group which reverts to normal iso-1-cytochrome c by a G · C → A · T transition. In addition, NQO produces a significant rate of reversion of all cyc1 mutants which revert by G · C transversions, e.g. the amber (UAG) mutant and the initiation mutants containing AGG, and probably CUG mutant codons. It did not revert the ochre mutant which contains no G · C base pairs. Ten NQO-induced revertants of the amber mutant cyc1-179 contained the expected replacements of residues of tyrosine, and ten NQO-induced revertants of each of the cyc1-131 and cyc1-133 initiation mutants all contained the expected normal iso-1-cytochrome c. The structures of these iso-1-cytochromes c and the pattern of reversion of the tester strains indicate that base-pair substitutions arise at G · C base pairs which are the site of NQO attack. Thus NQO induces G · C → A · T transitions, G · C → T · A transversions and possibly G · C → C · G transversions. Because of its mode of action, NQO may be useful in less-defined systems for identifying G · C base pairs in mutant codons.