DNA TOPOISOMERASE-I IS ESSENTIAL IN DROSOPHILA-MELANOGASTER

Both biochemical and genetic experiments suggest that the type I DNA t opoisomerase may participate in DNA replication, recombination, transc ription, and other aspects of DNA metabolism. Despite its apparent imp ortance, genetic studies in unicellular organisms including eubacteria and yeasts indicate that topoisomerase I is not essential for viabili ty. We have previously isolated the cDNA clone encoding DNA topoisomer ase I from Drosophila melanogaster. We report here the cytogenetic map ping of top1 to the X chromosome at 13C1 and isolation of top1 genomic DNA. Using P-element mutagenesis, we have isolated a mutant deficient in Drosophila topoisomerase I functions. Genetic studies of this muta nt show that topoisomerase I is essential for the growth and developme nt of the fruit fly, a multicellular organism. The biological function s of topoisomerase I are inferred from our analysis of the regulation of topoisomerase I expression during Drosophila development.