A PROMOTER IDENTIFIED IN THE 3'-END OF THE AC TRANSPOSON CAN BE ACTIVATED BY CIS-ACTING ELEMENTS IN TRANSGENIC ARABIDOPSIS LINES

In experiments directed to develop a promoter trap strategy in Arabido psis, using a Ds chimaeric element containing a promoterless beta-gluc uronidase (GUS) gene, we identified a promoter in the 3' end region of the Ac transposable element. The promoter initiates most of the trans cripts at coordinate 4250 in the Ac sequence and is oriented towards t he internal part of the element. When fused to a promoterless GUS gene , the promoter allows transient expression in Arabidopsis leaves. Afte r stable integration into the Arabidopsis genome, no GUS activity was observed in most of the transformed lines analysed. Only two of them e xhibited different tissue-specific GUS expression. When a CaMV 35S pro moter was introduced into the transformation vector, downstream to the reporter gene, a high level of GUS activity was observed in all the t ransformants. These results strongly suggest that the promoter is not normally expressed at a significant level in Arabidopsis transformed l ines except when activated by neighbouring cis-acting enhancer element s. This opens an interesting possibility for using this promoter to de velop 'enhancer trap' strategies in Arabidopsis. Since only one Ac tra nscript, initiating in the 5' end region of the element has been repor ted to date in maize, the putative biological function of the promoter remains an open question.