DNA topoisomerase I is essential in Drosophila melanogaster.

Both biochemical and genetic experiments suggest that the type I DNA topoisomerase may participate in DNA replication, recombination, transcription, and other aspects of DNA metabolism. Despite its apparent importance, genetic studies in unicellular organisms including eubacteria and yeasts indicate that topoisomerase I is not essential for viability. We have previously isolated the cDNA clone encoding DNA topoisomerase I from Drosophila melanogaster. We report here the cytogenetic mapping 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 mutant show that topoisomerase I is essential for the growth and development of the fruit fly, a multicellular organism. The biological functions of topoisomerase I are inferred from our analysis of the regulation of topoisomerase I expression during Drosophila development.