CHARACTERIZATION OF THE PUTATIVE TRANSPOSASE MESSENGER-RNA OF TAG1, WHICH IS UBIQUITOUSLY EXPRESSED IN ARABIDOPSIS AND CAN BE INDUCED BY AGROBACTERIUM-MEDIATED TRANSFORMATION WITH DTAG1 DNA

Tag1 is an autonomous transposable element of Arabidopsis thaliana. Ta g1 expression was examined in two ecotypes of Arabidopsis (Columbia an d No-0) that were transformed with CaMV 35S-Tag1-GUS DNA These ecotype s contain no endogenous Tag1 elements. A major 2.3-kb and several mino r transcripts were detected in ail major organs of the plants. The maj or transcript encoded a putative transposase of 84.2 kD with two nucle ar localization signal sequences and a region conserved among transpos ases of the Ac or hAT family of elements. The abundance of Tag1 transc ripts varied among transgenic lines and did not correlate with somatic excision frequency or germinal reversion rates, suggesting that facto rs other than transcript levels control Tag1 excision activity. In unt ransformed plants of the Landsberg ecotype, which contain two endogeno us Tag1 elements, no Tag1 transcripts were detected. Agrobacterium-med iated transformation of these Landsberg plants with a defective 1.4-kb Tag1 element resulted in the appearance of full-length Tag1 transcrip ts from the endogenous elements. Transformation with control DNA conta ining no Tag1 sequences did not activate endogenous Tag1 expression. T hese results indicate that Arobacterium-mediated transformation with d Tag1 can activate the expression of Tag1.