Characterisation of novel target promoters for the dexamethasone-inducible/tetracycline-repressible regulator TGV using luciferase and isopentenyl transferase as sensitive reporter genes

The chimeric transcriptional activator TGV mediates dexamethasone (dx)-indu cible and tetracycline (tc)-repressible transgene expression in tobacco (dx -on/ tc-off system). The expression profiles of four different synthetic ta rget promoters, comprising multiple TGV binding sites upstream of a core pr omoter, were characterised using the sensitive luciferase assay. Induction factors of over 1000 were measured in roots and leaves of over 30% of the t ransgenic plants, irrespective of the promoter used. Promoters P-TF and P-T ax, which carry the -48 to +1 region of the Cauliflower Mosaic Virus 35S pr omoter, showed higher expression levels in both the uninduced and induced s tates than P-Top10 and P-TFM, which harbour several point mutations in this region. Moreover, P-Tax expressed higher background activities than P-TF, indicating that the sequence of the synthetic regulatory region can influen ce background levels. The usefulness of the dx-on/tc-off system for experim ents addressing gene function was demonstrated by using it to control the e xpression of isopentenyl transferase. This enzyme catalyses the rate-limiti ng step in cytokinin biosynthesis and causes phenotypic effects even at low expression levels. Only dr-induced transgenic plants displayed phenotypic alterations indicative for increased cytokinin synthesis (e.g. outgrowth of lateral buds). Simultaneous treatment of selected buds with the anti-induc er tc suppressed bud growth. This result suggests that cytokinins cannot se rve as mobile signals to elicit the release of apical dominance in tissues compromised for enhanced cytokinin synthesis.