Tc7, a Tc1-hitch hiking transposon in Caenorhabditis elegans

We have found a novel transposon in the genome of Caenorhabditis elegans. Tc7 is a 921 bp element, made up of two 345 bp inverted repeats separated by a unique, internal sequence. Tc7 does not contain an open reading frame. The outer 38 bp of the inverted repeat show 36 matches with the outer 38 bp of Tc1. This region of Tc1 contains the Tc1-transposase binding site. Furthermore, Tc7 is flanked by TA dinucleotides, just like Tc1, which presumably correspond to the target duplication generated upon integration. Since Tc7 does not encode its own transposase but contains the Tc1-transposase binding site at its extremities, we tested the ability of Tc7 to jump upon forced expression of Tc1 transposase in somatic cells. Under these conditions Tc7 jumps at a frequency similar to Tc1. The target site choice of Tc7 is identical to that of Tc1. These data suggest that Tc7 shares with Tc1 all the sequences minimally required to parasitize upon the Tc1 transposition machinery. The genomic distribution of Tc7 shows a striking clustering on the X chromosome where two thirds of the elements (20 out of 33) are located. Related transposons in C. elegans do not show this asymmetric distribution.