Identification of UV-B light-responsive regions in the promoter of the tryptophan decarboxylase gene from Catharanthus roseus

The tryptophan decarboxylase (Tdc) gene encodes a key enzyme in the biosynt hesis of terpenoid indole alkaloids (TIAs) in Catharanthus roseus. TIAs abs orb ultraviolet light (UV) and putative functions in plants include a role as UV protectants. In support of this possible function we demonstrate here that UV light induces accumulation of several TIAs as well as expression o f the Tdc gene in C. roseus. In addition, in tobacco a Tdc-gusA construct w as found to be specifically induced by UV-B light. Lack of induction by UV- A or other wavelengths of light indicate that Tdc expression is regulated b y a specific UV-B receptor and corresponding signal transduction pathway. T o identify UV-responsive Tdc promoter elements, a loss-of-function analysis was performed, in which deletion derivatives were fused to the gusA report er gene and analysed in transgenic tobacco plants. Truncation of the Tdc pr omoter from -1818 (relative to the start of transcription) to -160 reduced expression levels two-fold without affecting the qualitative UV response. D eletion to -37 further reduced expression levels five-fold, but the Delta 3 7 promoter also remained UV-responsive. Subsequently, the -160 to -37 regio n was further studied by gain-of-function experiments, in which the transcr iptional activities of tetramerized subfragments fused to truncated promote rs were analysed. Combination of the data identified several functional reg ions in the -160 to +198 promoter. The -160 to -99 region acts as the main transcriptional enhancer. UV-responsive elements appeared to be redundant i n the -160 Tdc promoter and to reside between -99 and -37 and between -37 a nd +198.