High-frequency precise excision of the Drosophila foldback transposable element

Precise excision of transposable elements in prokaryotes is a rare event which occurs at a significantly lower rate than transposition and other element-mediated events1. Thus, we were intrigued by a eukaryotic transposable element which seemed capable of precise excision at high frequencies. The white-crimson (wc) mutation in Drosophila, a highly unstable allele of the X-linked eye colour locus, white2, resulted from the insertion of a member of the foldback (FB) transposable element family3,4. This mutation reverts to its parental phenotype at a frequency of greater than 1 in 103 X chromosomes2. Characterization of these revertants by Southern blots of genomic DNA indicated that they resulted from loss of the wc insertion3. Here we report the nucleotide sequence of the excision point in these revertants, and conclude that the FB element responsible for the wc mutation is capable of precise excision at high frequencies.