Differential utilization of alternate initiation sites in a plant defense gene responding to environmental stimuli

The upstream regulatory region of an efficiently expressed phenylalanine ammonia-lyase (PAL5) gene from tomato (Lycopersicon esculentum) was characterized by DNA sequencing and S1-nuclease-protection studies. Two transcription initiation sites giving rise to a long and a short transcript were identified, each corresponding to a different putative TATA sequence element with multiple conserved upstream regulatory motifs. When tomato plants were challenged with three different environmental stimuli, wounding, light and Verticillium infection, this gene was observed to utilize the alternate initiation sites differentially, in a tissue-dependent and stimulus-dependent manner. In general, basal levels of the shorter transcript were very low but preferentially and strongly stimulated by the environmental factors. In contrast, the longer transcript was less affected, essentially representing a constitutive expression of this gene. The results suggest that the multiple regulatory sites in the PAL5 gene promoter provide for both an essential constitutive expression of phenylalanine ammonia-lyase as well as an appropriate response to different environmental stresses.