2 G-BOX-RELATED SEQUENCES CONFER DIFFERENT EXPRESSION PATTERNS IN TRANSGENIC TOBACCO

We have analyzed the expression patterns conferred by two G-box-related motifs, a perfect palindromic sequence (PA, 5'-GCCACGTGGC-3') and motif I (Iwt, 5'-GTACGTGGCG-3'), in transgenic tobacco plants. A mutant version of motif I, Imu, was used as a negative control. PA is present in the promoters of several different genes, whereas Iwt is a conserved sequence found in abscisic acid-inducible promoters. Previously we have demonstrated that PA and Iwt, but not Imu, can bind to the tobacco transcription activator TAF-1 in vitro, with the PA sequence showing a 70-fold higher affinity as compared to Iwt. We found that tetramers of PA and Iwt, which differ by only 2 bp per 10-bp repeat, confer very different tissue-specific and expression patterns in transgenic tobacco plants. PA confers preferential expression in root tissues with a low level of activity in leaves, whereas Iwt directs developmentally regulated expression in seeds beginning 15 days after petals have fully expanded until seed maturation. Imu appears to be inactive because it gives the same expression pattern as the -90 cauliflower mosaic virus 35S promoter control. RNA gel blot analysis showed that the expression pattern of TAF-1 mRNA is similar to that directed by PA, suggesting that TAF-1 may be involved in the transcriptional regulation of PA.