A cauliflower mosaic virus (CaMV) 35S promoter derivative, which is tightly repressed by the Tn 10 encoded Tet repressor in a transient expression system as well as in transgenic plants has been constructed. After treatment of transgenic plants with tetracycline (Tc) the activity of the reporter enzyme beta-glucuronidase (GUS) increased up to 500-fold in tissue culture as well as under greenhouse conditions. Efficient de-repression was achieved by Tc uptake through the roots as well as by Tc treatment of leaves of intact plants. As Tc is not very stable in the plants, this system can also be used for a transient expression of a transgene. This system provides a unique tool for regenerating transgenic plants carrying a repressed transgene and for efficiently de-repressing its activity by a specific inducer at any time point of further development.