Somatic and germinal excision activities of the arabidopsis transposon Tag1 are controlled by distinct regulatory sequences within Tag1

Various sequences within Tag1, the endogenous transposon of Arabidopsis, we re examined to determine how Tag1 excision and expression are regulated. Th e 5' intron for the major 2.3-kb Tag1 transcript was found to be critical f or the accumulation of Tag1 transcripts and for high rates of somatic excis ion. This was true for the autonomous element in cauliflower mosaic virus 3 5S-Tag1-beta -glucuronidase constructs and for a two-component system using the 35S promoter to produce Tag1 transposase and a beta -glucuronidase::dT ag1 marker construct to score for excision. The 3' introns of Tag1, althoug h not needed for high transposase expression in primary transgenic plants, were important for maintaining high levels of somatic excision and accumula tion of the major but not the minor Tag1 transcripts in subsequent generati ons. With both 5' and 3' introns present, exchanging the 5' promoter region of Tag1 with the 35S promoter did not affect the timing of Tag1 excision s ignificantly, but it did disrupt germinal excision. Removal of the 5' intro n did not abolish germinal excision activity, however. These results indica te that somatic and germinal excision of Tag1 are differentially controlled , with the 5' promoter region being critical for germinal excision activity and the 5' intron playing an important role for somatic excision, possibly via intron-mediated enhancement.