Extrachromosomal copies of transposon Tc1 in the nematode Caenorhabditis elegans

Extrachromosomal copies of the 1.6-kilobase transposable element Tc1 are present at the level of between 0.1 and 1.0 copy per cell in Caenorhabditis elegans strain Bergerac. Extrachromosomal elements were detected and studied using Southern hybridizations employing a Tc1-specific probe. The amount of extrachromosomal Tc1 DNA was roughly constant during development in Bergerac, which has approximately 300 integrated chromosomal copies of Tc1 in its haploid genome. Extrachromosomal Tc1 DNA was not detected in strain Bristol, which has 30 chromosomal copies of Tc1. Three forms of extrachromosomal DNA were detected. The predominant form was a 1.6-kilobase linear molecule with ends corresponding to the ends of an integrated Tc1 element. The other two forms were, respectively, relaxed and supercoiled circular copies of the element. Structural assignments were based on electrophoretic mobility, the results of sedimentation velocity and equilibrium density gradient experiments, and on the sizes of the products produced by treatment of purified extrachromosomal DNA with restriction endonucleases. The suggestion is made that these extrachromosomal transposable elements are the products of excision events known to be occurring at high frequency in somatic cells in Bergerac.