Preferential cleavage of Paramecium DNA mediated by the C. elegans Tc1 transposase in vitro

In the ciliate Paramecium aurelia complex, thousands of internal eliminated sequences (IESs) are excised from the germline micronuclear DNA during macronuclear differentiation. Based on the resemblance of Paramecium IES end sequences to Tc1 transposon termini, it has been proposed that Paramecium IESs might have degenerately evolved from Tc1 family transposons, and still be removed by an enzyme homologous to a Tc1 transposase. In this study, we found that transposase preferentially cleaved (or nicked) 58 sites near the IESs in Paramecium DNA, at sequences consisting of TT or TCTA. Since one excision junction of the P. primaurelia W2 IES was included in such sites, this suggests that a Tc1-like transposase is involved in the IES excision process, although it is probably not a sole factor responsible for the precise cleavage. In addition, unmethylated substrate DNA appeared to decrease the cleavage specificity, suggesting an involvement of DNA methylation in the cleavage. Although these results do not directly address the transposon origin of Paramecium IESs, it is likely that the enzymatic machinery responsible for the initial cleavage is derived from a Tc1-like transposase. The mechanism necessary for precise excision is discussed, in relation to recent knowledge of IES excision obtained in Tetrahymena and Paramecium.