In, Saccharomyce8 cerevisiae, after 8-methoxypsoralen 18-(OMe)Ps] photoaddition, more crosslinks are induced per unit dose in mitochondrial DNA than in nuclear DNA. In wildtype cells treated.in the exponential phase of growth, single-and double-strand breaks are produced during crosslink removal and then are rejoined upon postexposure incubation. The incision step is almost blocked in the rad3-2 mutant, which is also defective in excision-repair of UV-induced (254 nm) pyrimidine dimers. The cutting of crosslinks from nuclear DNA is depressed in wild-type stationary-phase cells. This is correlated with a higher sensitivity of such cells to 8-(OMe)Ps photoinduced, cell killing. The incision of crosslinks is dramatically reduced in mitochondrial DNA. The rejoining ofsingle-and double-strand breaks is not only dependent on the product of the RAD51 gene (as shown by others) but also of the PS02 gene. A correlation was found.between the ability to recombine and strand rejoining. Therefore, as in bacteria, both the excision and the recombinational repair systems are. involved in crosslink repair in yeast. However, double-strand breaks in yeast constitute repair intermediates which are not detected in Eacherichia coli. The LD37 (dose necessary to induce a mean of one lethal hit per cell) corresponds to about 120 crosslinks per genome in.exponential-phase cells of the wild type and to 1-2 crosslinks in the p8o2-1 mutant.The psoralen derivative, 8-methoxypsoralen [8-(OMe)Ps] intercalates in DNA; upon exposure to near UV radiation, it reacts with pyrimidine bases to form monoadducts and interstrand crosslinks (1, 2). These photomodifications of DNA lead to lethality and induction of mutation and recombination (1-3). In addition, the production in yeast of the mitochondrial "petite" mutation (rho-) has been reported (4-6). Removal ofcrosslinks has been described in Escherichia coli (7-10) and in mammalian cells in culture (11,12). In E. coli, the excision and the recombinational repair pathways are involved in the restitution of an intact genome from DNA containing psoralen photoproducts (7,9,10). In eukaryotes, limited information is available on the molecular steps in relation to the genetic control of repair of crosslinks in DNA. On the basis of sensitivity to cell killing in rad mutants compared to the wild type, it was demonstrated that in the yeast Saccharomyces cerevisiae, in addition to the excision (rad3 type), the error-prone (rad6 type), and the DNA double-strand-break (rad52) repair pathways (5, 13), some repair steps controlled by a class of genes designated pso (14) also are implicated in the repair of this last type of lesion.In this report, we determined the proportion of 8-(OMe)Ps photoinduced crosslinks as a function ofdose and followed their fate in nuclear and mtDNA in a normal strain. The influence of the growth phases, known to modify the lethal response (15), on crosslink removal was studied. An excision-deficient mutant (rad3) and one of the pso mutants (pso2) defective in the G2-phase repai...