Tdd-4, a DNA transposon of Dictyostelium that encodes proteins similar to LTR retroelement integrases

Tdd-4 is the first DNA transposon to be isolated from Dictyostelium discoid eum, This element was isolated by insertion into a target plasmid, Two clas ses of elements were identified which include a 3.8 kb version and a 3.4 kb deleted version. Sequence analysis reveals that the 145 bp inverted termin al repeats contain the 5'-TG...CA-3' conserved terminal dinucleotides found in prokaryotic transposons and integrated LTR retroelement DNA sequences. Tdd-4 open reading frames are assembled by removal of six introns, introns 1-5 conform to the GT-AG rule, whereas intron 6 appears to be an AT-AA intr on, Also, intron 6 undergoes an alternative 5' splicing reaction. The alter natively spliced region encodes 15 tandem SPXX repeats that are proposed to function as a DNA binding motif, By analogy to other transposons that enco de two proteins from the same gene, the full-length Tdd-4 protein is the pu tative transposase and the truncated Tdd-4 protein is the putative transpos ition inhibitor. Protein database searches demonstrate Tdd-4 encoded protei ns are unique for a DNA element by containing similarities to retroviral/re trotransposon integrases. The putative Tdd-4 transposase contains the same structural relationship as integrases by possessing an N-terminal HHCC moti f, a central DDE motif and a C-terminal DNA-binding domain composed of the SPXX motif.