The RADI gene of Saccharomyces cerevisiae is required for the incision step of excision repair of damaged DNA. In this paper, we report our observations on the effect of the RADI gene on genetic recombination.Mitotic intrachromosomal and interchromosomal recombination in RAD+, radi, rad52, and other rad mutant strains was examined. The radl deletion mutation and some radl point mutations reduced the frequency of intrachromosomal recombination of a his3 duplication, in which one his3 allele is deleted at the 3' end while the other his3 allele is deleted at the 5' end. Mutations in the other excision repair genes, RAD2, RAD3, and RAD4, did not lower recombination frequencies in the his3 duplication. As expected, recombination between the his3 deletion alleles in the duplication was reduced in the rad52 mutant. The frequency of HIS3+ recombinants fell synergistically in the radl rad52 double mutant, indicating that the RADI and RAD52 genes affect this recombination via different pathways. In contrast to the effect of mutations in the RAD52 gene, mutations in the RADI gene did not lower intrachromosomal and interchromosomal recombination between heteroalleles that carry point mutations rather than partial deletions; however, the radlA mutation did lower the frequency of integration of lnear plasmids and DNA fragments into homologous genomic sequences. We suggest that RADI plays a role in recombination after the formation of the recombinogenic substrate.In the yeast Saccharomyces cerevisiae, 10 genes, RAD1, RAD2, RAD3, RAD4, RAD7, RADIO, RAD14, RAD16, RAD23, and MMS19, are known to be involved in excision repair of DNA damaged by UV light and by other agents that deform the DNA helix. The radi, rad2, rad3, rad4, and radiO mutants are highly sensitive to UV light and to other DNA-damaging agents and are totally defective in the incision of damaged DNA (18,27,36). The mms19, rad7, radI4, radi6, and rad23 mutants are not as sensitive to DNAdamaging agents and differ in excision proficiency from each other (18,19). In their absolute requirement for incision, the RADI, RAD2, RAD3, RAD4, and RADIO genes of S. cerevisiae resemble the Escherichia coli uvrA, uvrB, and uvrC genes. The UvrABC enzyme complex of E. coli cuts the damaged strand at two sites, which are 7 to 8 nucleotides 5' and 3 to 4 nucleotides 3' from the damage (29, 37). The UvrD and Poll enzymes catalyze the release of the UvrABC enzyme complex from the damaged site (4, 10).In contrast to the E. coli uvrA, uvrB, and uvrC genes, which are required only for incision of damaged DNA, some of the yeast incision genes seem to be involved in other cellular processes as well. The RAD3 gene, in addition to its role in incision, is essential for cell viability; rad3 deletion mutants are inviable (8, 21), and various rad3 mutants that are defective only in the excision repair function or that are temperature sensitive for growth but proficient in DNA repair exist (22, 23; R. Schiestl and S. Prakash, unpublished observations). The RAD3 protein has been purified in our lab...