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)-inducible and tetracycline (tc)-repressible transgene expression in tobacco (dx-on/ tc-off system). The expression profiles of four different synthetic target promoters, comprising multiple TGV binding sites upstream of a core promoter, were characterised using the sensitive luciferase assay. Induction factors of over 1,000 were measured in roots and leaves of over 30% of the transgenic plants, irrespective of the promoter used. Promoters PTF and PTax, which carry the -48 to +1 region of the Cauliflower Mosaic Virus 35S promoter, showed higher expression levels in both the uninduced and induced states than PTop10 and PTFM, which harbour several point mutations in this region. Moreover, PTax expressed higher background activities than PTF, indicating that the sequence of the synthetic regulatory region can influence background levels. The usefulness of the dx-on/tc-off system for experiments addressing gene function was demonstrated by using it to control the expression of isopentenyl transferase. This enzyme catalyses the rate-limiting step in cytokinin biosynthesis and causes phenotypic effects even at low expression levels. Only dx-induced transgenic plants displayed phenotypic alterations indicative for increased cytokinin synthesis (e.g. outgrowth of lateral buds). Simultaneous treatment of selected buds with the antiinducer tc suppressed bud growth. This result suggests that cytokinins cannot serve as mobile signals to elicit the release of apical dominance in tissues compromised for enhanced cytokinin synthesis.