Allelism of Saccharomyces cerevisiae genes PSO6, involved in survival after 3-CPs+UVA induced damage, and ERG3, encoding the enzyme sterol C-5 desaturase.

Sequencing of the yeast gene that complemented the sensitivity to the photoactivated monofunctional 3-carbethoxypsoralen of the pso6-1 mutant strain revealed that the ERG3 locus, encoding sterol C-5 desaturase involved in biosynthesis of ergosterol, is allelic to PSO6. Disruption of the ERG3 gene yielded an erg3Delta mutant viable in ergosterol-containing YEPD media with the same pleiotropic mutant phenotype known for pso6-1 and erg3 mutants, including sensitivity to hydrogen peroxide and paraquat. Thus, the erg3/pso6 yeast mutant seems to be more sensitive than the WT to 3-CPs+UVA because of the oxidative damage contributed by this treatment and not because of an impaired repair of the furocoumarin-thymine monoadducts formed in the DNA. We found a significant increase of petites amongst erg3Delta and pso6-1 yeast mutant strains grown in conditions where respiration was mandatory. Mutant pso6-1, with its lowered content of ergosterol, exhibited enhanced synthesis of chitin that was maldistributed and not confined to the bud scars. Chitin overproduction in pso6/erg3 mutants resulted in hypersensitivity to Calcofluor White.