TAG1 IS AN AUTONOMOUS TRANSPOSABLE ELEMENT THAT SHOWS SOMATIC EXCISION IN BOTH ARABIDOPSIS AND TOBACCO

Tag1 is a transposable element first identified as an insertion in the CHL1 gene of Arabidopsis. The chl1::Tag1 mutant originated from a pla nt (ecotype Landsberg erecta) that had been transformed with the maize transposon Activator (Ac), which is distantly related to Tag1, Genomi c analysis of untransformed Landsberg erecta plants demonstrated that two identical Tag1 elements are present in the Landsberg erecta genome , To determine what provides transposase function for Tag1 transpositi on, we examined Tag1 excision in different genetic backgrounds, First, the chl1::Tag1 mutant was backcrossed to untransformed wild-type Arab idopsis plants to remove the Ac element(s) from the genome. F-2 progen y that had no Ac elements but still retained Tag1 in the CHL1 gene wer e identified. Tag1 still excised in these Ac-minus progeny producing C HL1 revertants; therefore, Ac is not required for Tag1 excision, Next, Tag1 was inserted between a cauliflower mosaic virus 35S promoter and a beta-glucuronidase (GUS) marker gene and transformed into tobacco. Transformants showed blue-staining sectors indicative of Tag1 excision . Transgenic tobacco containing a defective Tag1 element, which was co nstructed in vitro by deleting an internal 1.4-kb EcoRI fragment, did not show blue-staining sectors, We conclude that Tag1 is an autonomous element capable of independent excision, The 35S-GUS::Tag1 construct was then introduced into Arabidopsis. Blue-staining sectors were found in cotyledons, leaves, and roots, showing that Tag1 undergoes somatic excision during vegetative development in its native host.