In the Drosophila flightless mutant Ifin(3)3, a transposable element inserted into the alternatively spliced fourth exon of the tropomyosin I (TmI) gene prevents proper expression of Ifm-TmI, the tropomyosin isoform found in indirect flight muscle. We have rescued the flightless phenotype of Ifin(3)3 flies using P-element-mediated transformation with a segment of the Drosophia genome containing the wild-type TmI gene plus 2.5 kb of 5' flanking and 2 kb of 3' flanking DNA. The inserted TmI gene is expressed with the proper developmental and tissue specificity, although its level of expression vanes among the five transfonned lines examined. These conclusions are based on analyses of flight, myofibrillar morphology, and TmI RNA and protein levels. A minimum of two copies of the inserted TmI gene per cell is necessary to restore flight to most of the flies in each line. We also show that the Ifm-TmI isoform is expressed in the leg muscle of wild-type flies and is decreased in Ifn(3)3 leg muscle. Homozygous Ifin(3)3 mutants do not jump. The ability to jump can be restored with a single copy of the wild-type TmI gene per cell.

We constructed strains of Saccharomyces cerevisiae that contained two different mutant alleles of either the leu2 gene or the ura3 gene. These repeated genes were located on nonhomologous chromosomes; the two ura3- alleles were located on chromosomes V and XII and the two leu2- alleles were located on chromosomes III and XII. Genetic interactions between the two mutant copies of a gene were detected by the generation of either Leu+ or Ura+ revertants. Both spontaneous and ultraviolet irradiation-induced revertants were examined. By genetic and physical analysis, we have shown that Leu+ or Ura+ revertants can arise by a variety of different genetic interactions. The most common type of genetic interaction is the nonreciprocal transfer of information from one repeat to the other. We also detected reciprocal recombination between repeated genes, resulting in reciprocally translocated chromosomes.