FUNCTIONAL DISSECTION OF A BEAN CHALCONE SYNTHASE GENE PROMOTER IN TRANSGENIC TOBACCO PLANTS REVEALS SEQUENCE MOTIFS ESSENTIAL FOR FLORAL EXPRESSION

Expression of chalcone synthase (CHS), the first enzyme in the flavono id branch of the phenylpropanoid biosynthetic pathway in plants, is in duced by developmental cues and environmental stimuli. We used plant t ransformation technology to delineate the functional structure of the French bean CHS15 gene promoter during plant development. In the absen ce of an efficient transformation procedure for bean, Nicotiana tabacu m was used as the model plant. CHS15 promoter activity, evaluated by m easurements of beta-D-glucuronidase (GUS) activity, revealed a tissue- specific pattern of expression similar to that reported for CHS genes in bean. GUS activity was observed in flowers and root tips. Floral ex pression was confined to the pigmented part of petals and was induced in a transient fashion. Fine mapping of promoter cis-elements was acco mplished using a set of promoter mutants generated by unidirectional d eletions or by site-directed mutagenesis. Maximal floral and root-spec ific expression was found to require sequence elements located on both sides of the TATA-box. Two adjacent sequence motifs, the G-box (CACGT G) and H-box (CCTACC(N)(7)CT) located near the TATA-box, were both ess ential for floral expression, and were also found to be important for root-specific expression. The CHS15 promoter is regulated by a complex interplay between different cis-elements and their cognate factors. T he conservation of both the G-box and H-box in different CHS promoters emphasizes their importance as regulatory motifs.