Reversion of nonsense mutants induced by 4-nitroquinoline-1-oxide in Schizosaccharomyces pombe

We have studied the reversion of 8 nonsense alleles located in 7 different genes of Schizosaccharomyces pombe using 4-nitroquinoline-1-oxide (NQO) as a mutagenic agent. The nonsense mutants of S. pombe have been classified according to their suppressibility by defined opal and ochre suppressors into a class of efficiently suppressed opal and a class of inefficiency suppressed ochre mutants. The UGA alleles tested all revert consistently with NQO, in agreement with the high specificity of this mutagen for G-residues reported for bacteria and yeast. The UAA alleles show a lack or a low level of reversion with NQO. This low level of reversion is due to the low level of non-G-specific transversions at A sites of the UAA triplet. Within each class of nonsense mutants the extent of induction is site-dependent. We conclude that NQO acts predominantly on G-residues in S. pombe.