The pso mutants of Saccharomyces cerevisiae comprise two groups: one deficient in DNA repair and another with altered mutagen metabolism.

Yeast mutants that are sensitive to photoactivated psoralens, named pso mutants, were isolated and described more than 20 years ago. Nine genes responsible for the pso phenotypes were identified and seven of them cloned and molecularly characterized. Of the nine PSO genes of yeast seven apparently encode proteins involved in the repair of DNA lesions generated by photoinduced psoralens and by other mutagens, while two, PSO6 and PSO7, are responsible for structural elements of the membrane and for a functional respiratory chain, respectively. Of the seven proven or putative DNA repair genes six directly or indirectly control induced mutagenesis. Four of these PSO loci were found allelic to already known repair genes, whereas two, PSO2 and PSO4, represent new genes involved in DNA repair and in repair/pre-mRNA processing in S. cerevisiae. Gene PSO2 encodes a protein indispensable for repair of DNA interstrand cross-links that are produced by a variety of bi- and poly-functional mutagens and that appears to be important for a likewise repair function in humans as well.