Structure and Function of Tpn transposable elements in the Japanese morning glory

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Some mutations of the Japanese morning glory were known to be unstable somatically and germinally as caused by insertions of transposable elements. Until now, transposable elements that cause mutable phenotypes have been isolated from several genes. All of these elements belong to the En/Spm family by their structures. They all carried common 28-bp terminal inverted repeats (TIRs) and subterminal regions with many repeats. These transposable elements with common terminal sequences are termed Tpn1 family. All of them thought to be nonautonomous elements and these elements in mutable alleles are mobilized by autonoumous elements. The copy number of Tpn1 family was estimated between 500 and 1000 copies per haploid genome. Using probe of subterminal regions, we isolated more than 200 Tpn clones, and 128 clones were classified into 30 groups by their restriction maps. We decided the complete sequences of 19 representative Tpn clones from 30 groups. The comparison of their common regions among all Tpns indicates that the 5'-common regions including TIR, subterminal and non-repetitive regions are much longer than 3'-common regions. Interestingly, their internal sequences of Tpns were quite different among groups and were derived from host gene sequences. Another interesting feature of the Tpn1 family is the phylogenetic trees of each 5' and 3'common sequences did not make parallel relationships. By the linkage between 5'-subterminal regions and internal sequences, recombination events between different groups occurred in 3'-regions. We deduced a mechanism that Tpns incorporated host gene sequences in process of evolution from these findings. 3' protruding ends of Tpns mainly invades into homologous host genes as templates when Abortive Gap Repair events were occurred.