THE EVOLUTIONARY ANALYSIS OF THE TNT1 RETROTRANSPOSON IN NICOTIANA SPECIES REVEALS THE HIGH VARIABILITY OF ITS REGULATORY SEQUENCES

We studied the evolution of the tobacco Tnt1 retrotransposon by analyz ing Tnt1 partial sequences containing both coding domains and U3 regul atory sequences obtained from a number of Nicotiana species. We detect ed three different subfamilies of Tnt1 elements, Tnt1A, Tnt1B, and Tnt 1C, that differ completely in their U3 regions but share conserved fla nking coding and LTR regions. U3 divergence between the three subfamil ies is found in the region that contains the regulatory sequences that control the expression of the well-characterized Tnt1-94 element. Thi s suggests that expression of the three Tnt1 subfamilies might be diff erently regulated. The three Tnt1 subfamilies were present in the Nico tiana genome at the time of species divergence, but have evolved indep endently since then in the different genomes. Each Tnt1 subfamily seem s to have conserved its ability to transpose in a limited and differen t number of Nicotiana species. Our results illustrate the high variabi lity of Tnt1 regulatory sequences. We propose that this high sequence variability could allow these elements to evolve regulatory mechanisms in order to optimize their coexistence with their host genome.