The denV gene of bacteriophage T4 was fused to a Drosophila hsp70 (70-kDa heat shock protein) promoter and introduced into the germ line of Drosophila by Pelement-mediated transformation. The protein product of that gene (endonuclease V) was detected in extracts of heat-shocked transformants with both enzymological and immunoblotting procedures. That protein restores both excision repair and UV resistance to mei-9 and mus201 mutants ofthis organism. These results reveal that the denV gene can compensate for excisionrepair defects in two very different eukaryotic mutants, in that the mus201 mutants are typical of excision-deficient mutants in other organisms, whereas the mei-9 mutants exhibit a broad pleiotropism that includes a strong meiotic deficiency. This study permits an extension of the molecular analysis of DNA repair to the germ line of higher eukaryotes. It also provides a model system for future investigations of other wellcharacterized microbial repair genes on DNA damage in the germ line of this metazoan organism.Although the mechanism of excision repair is now well understood in prokaryotes (1, 2), it remains obscure in eukaryotes in spite of the fact that numerous essential genes have been identified. In humans at least nine genes are necessary for the initial incision step of excision repair, and five are required in yeast (2). The well-characterized prokaryotic repair functions can, therefore, be employed as tools to investigate eukaryotic repair by testing their capacity to complement eukaryotic repair-deficient mutants. This approach has proven particularly fruitful with the denV gene of bacteriophage T4 (3, 4) as a result of the capacity of its single small protein product (endonuclease V) to complete the initial step of excision repair. That gene has been shown to restore the repair capacity of excision-deficient cell lines in Escherichia coli, yeast, hamster, and man (5-11).The availability of P-element-mediated transformation in Drosophila has made it possible to extend that approach to an intact metazoan organism. Using that technique we have introduced the denV gene into the genomes of two very different excision-deficient mutants of Drosophila, mus201 and mei-9, each of which is defective in the incision step of excision repair (12,13). Mutants at the mus201 locus are typical of excision-deficient mutants in other organisms (14) in that they are hypersensitive to killing by UV irradiation but are relatively insensitive to x-rays (14) and do not influence meiosis (13). In contrast, the mei-9 mutants are unique among excision-deficient mutants in any organism because they are not only hypersensitive to chemical carcinogens and nonionizing radiation but also sensitive to ionizing radiation (14). In addition, the mei-9 mutants exhibit defects in meiotic recombination, mismatch repair, and somatic chromosomal stability (14).MATERIALS AND METHODS Drosophila Mutants and Culture Conditions. The transformants generated in this study are designated P[(denV, neo)AJ1 through P[(denV, neo)A]...