TC1 TRANSPOSASE OF CAENORHABDITIS-ELEGANS IS AN ENDONUCLEASE WITH A BIPARTITE DNA-BINDING DOMAIN

The Tc1 transposon of Caeitorhabditis elegans is a member of the Tc1/mariner family of mobile elements. These elements have inverted terminal repeats that flank a single transposase gene. Here we show that Tc1 transposase, Tc1A, has a bipartite DNA binding domain related to the paired domain of mammalian and Drosophila genes. Both the DNA binding domain of Tc1A and the DNA binding site in the inverted repeat of Tc1 can be divided into two subdomains. Methylation interference studies demonstrate adjacent minor and major groove contacts at the inner part of the binding site by the N-terminal 68 amino acids of the DNA binding domain. In addition, Tc1A amino acids 69-142 are essential for major groove contacts at the outer part of the binding site. Recombinant Tc1A is found to be able to introduce a single strand nick at the 5' end of the transposon in vitro. Furthermore, Tc1A can mediate a phosphoryl transfer reaction. A mutation in a DDE motif abolishes both endonucleolytic and phosphoryl transfer activities, suggesting that Tc1A carries a catalytic core common to retroviral integrases and IS transposases.