Retrotransposons of the Tnt1B family are mobile in Nicotiana plumbaginifolia and can induce alternative splicing of the host gene upon insertion

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Abstract
Active retrotransposons have been identified in Nicotiana plumbaginifolia by their ability to disrupt the nitrate reductase gene in chlorate–resistant mutants selected from protoplast–derived cultures. In mutants E23 and F97, two independent insertions of Tnp2, a new retrotransposon closely related to the tobacco Tnt1 elements, were detected in the nitrate reductase gene. These two Tnp2 elements are members of the Tnt1B subfamily which shows that Tnt1B elements can be active and mutagenic in the N. plumbaginifolia genome. Furthermore, these results suggest that Tnt1B is the most active family of Tnt1 elements in N. plumbaginifolia, whereas in tobacco only members of the Tnt1A subfamily were found inserted in the nitrate reductase gene. The transcriptional regulations of Tnp2 and Tnt1A elements are most probably different due to non–conserved U3 regions. Our results thus support the hypothesis that different Nicotiana species contain different active Tnt1 subfamilies and that only one active Tnt1 subfamily might be maintained in each of these species. The Tnp2 insertion found in the F97 mutant was found to be spliced out of the nitrate reductase mRNA by activation of cryptic donor and acceptor sites in the nitrate reductase and the Tnp2 sequences respectively.