The origin of footprints of the Tc1 transposon of Caenorhabditis elegans.

Mutations caused by the Tc1 transposon in Caenorhabditis elegans can revert by loss of the element. Usually the transposon leaves behind a 'footprint'--a few nucleotides of one or both ends of the transposon. Two possible explanations for the footprints are: (i) imprecise excision or (ii) interrupted repair. Here I report that in a diploid animal having a homozygous Tc1 insertion the reversion frequency is approximately 10(-4), and a Tc1 footprint is found; however when the corresponding sequence on the homologous chromosome is wild-type, the reversion frequency is 100 times higher, and the reverted sequence is precise. Apparently the footprint results from incomplete gene conversion from the homologous chromosome, and not from imprecise excision of Tc1. These results support the following model: Tc1 excision leaves a double-strand DNA break, which can be repaired using the homologous chromosome or sister chromatid as a template. In heterozygotes repair can lead to reversion; in homozygotes Tc1 is copied into the 'empty' site, and only rare interrupted repair leads to reversion, hence the 100-fold lower reversion rate and the footprint.