THE TISSUE-SPECIFIC EXPRESSION OF A TOBACCO PHYTOCHROME-B GENE

We have isolated a genomic clone from Nicotiana tabacum, designated Nt -PHYB-1, encoding a type-II, ''green tissue'' phytochrome apoprotein. Recombinant genes, consisting of the 3319-bp promoter of the Nt-PHYB-1 gene (including the entire 5' untranslated sequence but not the ATC) or its deletion derivatives and the bacterial beta-glucuronidase repor ter gene, were constructed and transferred into tobacco. The expressio n patterns and levels of the endogenous Nt-PHYB-1, as well as those of the transgenes, were. determined by RNase protection assays and by be ta-glucuronidase histochemical staining. We show that (a) the PHYB-1 g ene has three transcription start sites, (b) the abundance of the thre e PHYB-1-specific mRNAs is different, and that (c) it is not regulated by light. However, we do demonstrate that transcription of the endoge nous PHYB-1 gene and that of the recombinant genes exhibit a well-defi ned organ and tissue specificity. This tobacco PHYB gene is relatively highly expressed in leaf, stem, and different floral organs but not i n root. Deletion analysis of the Nt-PHYB-1 promoter indicates that a 3 82-bp region, located between -1472 and -1089, is required for high-le vel expression of this gene.