Regulation of Tcl transposable elements in Caenorhabditis elegans.

C. elegans strains contain variable numbers of a 1.6-kb transposable genetic element. Activity of this element, which is denoted Tcl, shows regulation at at least two levels. At one level, excision of Tcl elements occurs in somatic cells at a frequency several orders of magnitude higher than in germ cells. Evidence is presented suggesting that this results from regulation at the level of trans-acting functions that are required for excision or that repress excision. At the second level, germ line transposition of Tcl occurs at greater frequency in some strains than in others. The hypothesis is proposed that this is because Tcl is one component of a two-element system, the second element of which differs between strains. Evidence for a second putative transposable element family in C. elegans is presented. This family has properties that suggest a relationship to Tcl. This possibility is currently under investigation.