Tsessebe, Topi and Tiang: three distinct Tc1-like transposable elements inthe malaria vector, Anopheles gambiae

Three distinct types of Tc1-family transposable elements have been identifi ed in the malaria vector, Anopheles gambiae. These three elements, named Ts essebe, Topi and Tiang, have the potential to encode transposases that reta in most of the conserved amino acids that are characteristic of this transp oson family. However, all three are diverged from each other by more than 5 0% at the nucleotide level. Full-length genomic clones of two types, Topi a nd Tsessebe, have been isolated and fully sequenced. The third, Tiang, is r epresented only by a 270 bp, PCR-amplified fragment of the transposase codi ng region. The Topi and Tsessebe elements are 1.4 kb and 2.0 kb in length, respectively, and differ in the length of their inverted terminal repeats ( ITRs). The Topi elements have 26 bp ITRs, whereas the Tsessebe clones have long ITRs ranging in length from 105 to 209 bp, with the consensus being ab out 180 bp. This difference is due primarily to variation in the length of an internal stretch of GT repeats. The copy number and location of these el ements in ovarian nurse cell polytene chromosomes varies greatly between el ement subtypes: Topi elements are found at between 17-31 sites, Tsessebe at 9-13 and at 20 euchromatic sites, in addition to several copies of these e lements in heterochromatic DNA. The copy number and genomic insertion sites of these transposons varies between A.gambiae strains and between member s pecies of the A.gambiae complex. This may be indicative of transpositionall y active Tc1-like elements within the genome.