The complete nucleotide sequence was determined for Tc1, a transposable element in the nematode Caenorhabditis elegans. The 1610-base-pair element terminates in 54-base-pair perfect inverted repeats and is flanked by a 2-base-pair duplication of the target sequence. The Tc1 sequence contains two long open reading frames on the same DNA strand but in different translational reading frames. The positions of transcriptional control sequences suggest that a single transcript is made, which could produce two polypeptides, 273 and 112 amino acids in length. These features, i.e. terminal repeats, target site duplication and open reading frames, make Tc1 similar to transposable elements from other species.

A transposable element, designated Tcl, has been characterized in Caenorhabditis elegans. Tcl is 1.7 kilobases long, has an inverted terminal repeat of <100 base pairs, and is repeated as a highly conserved element. The copy number and genomic positions of Tcl are extremely variable among strains, implying that Tcl is mobile. However, progeny of interstrain crosses did not show hybrid dysgenic traits that might be due to Tcl transposition. (8), which showed that the copy numbers and sites of these elements are hypervariable among different strains of the same species and, to a lesser degree, among individuals of the same strain (4, 9-11). Direct evidence for transposition of Ty, copia, and P elements came from analyses of mutations caused by their insertions into nonhomologous genomic sites (6, 12). The P elements are exceptional for their remarkably high frequency of transposition during interbreeding of certain Drosophila strains (13). Transposition of P elements is the most likely basis of P-M hybrid dysgenesis, in which progeny of hybrid crosses exhibit sterility, high rates of mutation, and chromosomal aberrations (14).In this paper, we report the characterization of a transposable element in the nematode Caenorhabditis elegans and compare it with other known eukaryotic transposable elements. Previous studies in this laboratory have shown that the two strains of C. elegans, Bristol and Bergerac, give occasional differences in restriction endonuclease cleavage patterns on Southern blots when probed with randomly selected cloned fragments (15). One such DNA polymorphism was found to be a 1.7-kilobasepair (kb) difference adjacent to the actin gene cluster (16). Here, we demonstrate that the observed polymorphism near the actin genes is due to the presence of the transposable element "Tcl" at that site in the Bergerac strain. Another polymorphism between the Bristol and Bergerac strains has been characterized by Emmons et al. (17) and it also was found to be due to Tci. MATERIALS AND METHODSNematodes. All C. elegans var. Bristol worms used in this study are descendants of a single Bristol N2 hermaphrodite (18

The target sequences for two independent insertions of the transposable element Tc1 from Caenorhabditis elegans show homology. Because both insertions are at palindromic TA/AT sequences, the exact boundaries of Tc1 cannot be distinguished; Tc1 could be 1610 bp and flanked by a 2-bp duplication of the target site or it could be 1612 bp and without target site duplication. The latter possibility implies a novel manner for insertion of a transposable element.