Tissue-specific expression of as-1 in transgenic tobacco

When integrated as a transgene in one or a few copies, the - 90 35S promoter of cauliflower mosaic virus confers expression in roots with little or no expression in cotyledons and leaves. The responsible cis element, activation sequence-1 (as-1) can bind to the nuclear factor ASF-1 as well as to the transcription factor TGA1a. Here, we show that microinjection of 10₄ molecules of TGA1a per cotyledon cell activated transgenes containing as-1-linked promoters. Transgenes with promoters linked to the octopine synthase (ocs) element, which also binds TGA1a, responded similarly. The acidic, N-terminal segment of TGA1a is important for transcription activation in vivo because a deletion mutant without the first 80 amino acids was inactive. Finally, we show that the -90 35S-β-glucuronidase (GUS) fusion gene conferred GUS expression in cotyledon cells when injected at 50,000 copies per cell. Collectively, these results provide support for the hypothesis that the undetectable expression of the as-1-linked transgene in cotyledon cells is most likely a result of its inability to compete for a limiting amount of its cognate transcription factor(s), presumably TGA1a or related proteins.