Transcription of tobacco phytochrome-A genes initiates at multiple start sites and requires multiple cis-acting regulatory elements.

Promoter regions of the Nicotiana tabacum PHYA1 and PHYA2 genes display 89% sequence identity. Analysis of the 5' ends of both the PHYA1 and the PHYA2 transcript revealed multiple, distinct mRNA species, each differing in length and in abundance. The levels of the major PHYA1, PHYA2 transcripts were found to be auto-regulated by phytochrome. This auto-regulation was most efficient in 2-8-day old seedlings. Furthermore, we examined the expression pattern of the PHYA1-GUS reporter gene, containing a 4700 bp PHYA1 promoter fragment or its 5'-deletion derivatives, by GUS histochemistry and by RNase protection assays in transgenic tobacco plants. Our data indicate that the PHYA1 promoter contains three regions which are necessary for the maximum level and regulated expression. We show that a 264 bp promoter fragment contains a cis-regulatory element(s) responsible for expression in the root tips of transgenic seedlings. The major cis-regulatory elements required for high-level transcription and expression in other organs are located in separate regions of the PHYA1 promoter. These data indicate the contribution of multiple cis-regulatory elements for the maximum and regulated expression of tobacco genes coding for phytochrome A protein.