Mutant allele pso7-1, that sensitizes Saccharomyces cerevisiae to photoactivated psoralen, is allelic with COX11, encoding a protein indispensable for a functional cytochrome c oxidase

The yeast gene PSO7 was cloned from a genomic library by complementation of the pso7-1 mutant's sensitivity phenotype to 4-nitroquinoline-1-oxide (4NQ O). Sequence analysis revealed that PSO7 is allelic to the 1.1-kb ORF of th e yeast gene COX11 which is located on chromosome XVI and encodes a protein of 28-kDa localized in the inner mitochondrial membrane. Allelism of PSO7/ COX11 was verified by non-complementation of 4NQO-sensitivity in diploids h omo- and hetero-allelic for the pso7-1 and cox11::TRP1 mutant alleles. Sens itivity to 4NQO was the same in exponentially growing cells of the pso7-1 m utant and the cox11::TRP1 disruptant. Allelism of COX11 and PSO7 indicates that the pso7 mutant's sensitivity to photoactivated 3-carbethoxypsoralen a nd to 4NQO is not caused by defective DNA repair, but rather is due to an a ltered metabolism of the pro-mutagen 4NQO in the absence of cytochrome oxid ase (Cox) in pso7-1/ cos11::TRP1 mutants/disruptants. Lack of Cox might als o lead to a higher reactivity of the active oxygen species produced by phot oactivated 3-carbethoxypsoralen. The metabolic state of the cells is import ant for their sensitivity phenotype since the largest enhancement of sensit ivity to 4NQO between wild-type (WT) and the pso7 mutant occurs in exponent ially growing cells, while cells in stationary phase or growing cells in ph osphate buffer have the same 4NQO resistance, irrespective of their WT/muta nt status. Strains containing the pso7-1 or cos11::TRP1 mutant allele were also sensitive to the oxidative stress-generating agents H2O2 and paraquat. Mutant pso7-1, as well as disruptant cos11::TRP1, harboured mitochondria t hat in comparison to WT contained less than 5% and no detectable Cox activi ty, respectively.