PhyB of Tobacco, a New Member of the Phytochrome Family

Abstract

Phytochrome plays a central role in the light-dependent regulation of photomorphogenesis such as seed germination, stem growth, and floral induction as well as chloroplast development and anthocyanin synthesis. This photoreceptor has been shown to modulate the pattems of expression for many plant genes (Nagy et al., 1988; Quail, 1991). Plants synthesize the physiologically inactive, red-light absorbing form Pr (A,, = 660 nm), and the absorption of red light converts these molecules to the far red-light absorbing form Pfr (A,, = 730 nm), which then triggers the physiological response. The presence of two distinct phytochrome pools in higher plants was demonstrated using spectroscopic and immunochemical methods as well as protein microsequencing (Tokuhisha et al., 1983; Abe et al., 1985). ”Pool 1,“ highly abundant in etiolated tissue, is rapidly degraded after redlight absorption. It is called “type 1” or “etiolated-tissue” phytochrome. Although ‘pool2” accounts for only 1% of the total phytochrome content in etiolated tissue, it forms the major phytochrome fraction in green tissue, because it is stable after photoconversion to the Pfr form. Consequently, it is called “type 2” or “green-tissue” phytochrome. Molecular cloning revealed cDNA clones coding for different phytochrome proteins in Arabidopsis thaliana (Sharrock and Quail, 1989), rice (Dehesh et al., 1991), and potato (Heyer and Gatz, 1992), and these clones were called p h y A , phyB, and phyC. Antibodies raised against expressed portions of different phytochrome cDNAs were used to study the abundance of the phytochrome species in physiologically well-characterized phytochrome mutants and were compared with wild-type plants. The results strongly imply that the p h y A gene codes for the light-labile, type 1 phytochrome and phyB encodes for type 2 phytochrome (Nagatani et al., 1991; Somers et al., 1991; Lopez-Juez et al., 1992). We have identified and characterized two tobacco (Nicotiana tabacum) cDNA clones (AO, A2) and one genomic clone (G8.2). Sequence analysis revealed that they show a high homology to the phyB gene of Arabidopsis. Both cDNAs have incomplete 5’ ends, whereas the genomic clone contains a translation initiation codon followed by a long open reading frame ending in an exon/intron border. The sequences were