Recombinational and mutagenic repair of psoralen interstrand cross-links in Saccharomyces cerevisiae

Psoralen photoreacts with DNA to form interstrand cross-links, which can be repaired by both nonmutagenic nucleotide excision repair and recombination al repair pathways and by mutagenic pathways. In the yeast Saccharomyces ce revisiae, psoralen cross-links are processed by nucleotide excision repair to form double-strand breaks (DSBs). In yeast, DSBs are repaired primarily by homologous recombination, predicting that cross-link and DSB repair shou ld induce similar recombination end points. We compared psoralen cross-link , psoralen monoadduct, and DSB repair using plasmid substrates with site-sp ecific lesions and measured the patterns of gene conversion, crossing over, and targeted mutation. Psoralen cross-link induced both recombination and mutations, whereas DSBs induced only recombination, and monoadducts were ne ither recombinogenic nor mutagenic. Although the cross-link- and DSB-induce d patterns of plasmid integration and gene conversion were similar in most respects, they showed opposite asymmetries in their unidirectional conversi on tracts: primarily upstream from the damage site for cross-links but down stream for DSBs. Cross-links induced targeted mutations in 5% of the repair ed plasmids; all were base substitutions, primarily T --> C transitions. Th e major pathway of psoralen cross-link repair in yeast is error-free and in volves the formation of DSB intermediates followed by homologous recombinat ion. A fraction of the cross-links enter an error-prone pathway, resulting in mutations at the damage site.