Two classes of cis sequences contribute to tissue-specific expression of a PAL2 promoter in transgenic tobacco

Genes encoding phenylalanine ammonia-lyase (PAL) are expressed in a complex pattern during plant development and in response to light, pathogen ingress, mechanical damage and other stresses. Analysis of the promoter of the bean PAL2 gene in transgenic tobacco has shown that some regions responsible for developmental expression are functionally compensatory. The minimum sequence containing all cis sequences necessary for developmental patterns of expression is within -254 bp of the transcription start site. Footprinting and electrophoretic mobility shift assay studies of this region revealed potential cis sequences which coincided with the functional domains defined by small deletions and promoter fusions. Mutations in these potential cis sequences in the context of the minimal -254 bp promoter altered tissue-specific expression patterns, confirming the importance of these sequences for expression in vivo. A functional model for the promoter is presented which predicts that three AC-elements, which are possible Myb protein binding sites, together with a G-box, interact to direct the complex patterns of tissue-specific expression observed.