Prokaryotes and phytochrome. The connection to chromophores and signaling

Prokaryotic systems have been important in phytochrome studies on several different levels. Bilins from cyanobacterial phycobiliproteins have allowed the production of recombinant holophytochrome and have provided insights into the attachment and functioning of the chromophore, while the recent discovery of functional phytochromes in the cyanobacterium Synechocystis and other prokaryotes has catalyzed work in the field. Synechocystis phytochrome is useful experimentally and, by making the modular structure and potential biochemical functions of phytochromes clearer, has provided an improved focus and new viewpoints for research. Some of the earliest studies of photobiology concerned phenomena in cyanobacteria: the complementary chromatic adaptation (CCA) of photosynthetic pigments to the light environment was first described in Engelmann’s laboratory in Berlin a century ago. Numerous other effects such as phototaxis, photoperiodism, cell division, and differentiation are also regulated by light in cyanobacteria. Plant plastids probably evolved from endosymbiotic cyanobacteria whose genes gradually moved to the host nucleus. There is thus every reason to expect evolutionary relationships between photoperception systems in cyanobacteria and plants. Detailed information about cyanobacterial photoreceptors was lacking, however, until genomic sequencing revealed a cyanobacterial phytochrome: ironically, the ease with which molecular methods can be used in prokaryotes has now turned the tables, with the cyanobacterial model providing a wealth of new ideas about the origins of phytochrome and its mode of action. Here we review the different ways in which cyanobacteria and other prokaryotes have contributed to research into plant photomorphogenesis and the phytochrome system (for review, see Elich and Chory, 1997; Quail, 1997a; Pepper, 1998). Phytochrome is an ubiquitous plant photoreceptor that was first characterized in the late 50s in relation to its peculiar photochromic behavior in red and far-red light (Butler et al., 1959). Phytochromes carry an open-chain tetrapyrrole (bilin) chromophore, which the apoprotein autocatalytically attaches to a conserved C residue (#380 in our alignment) via a Schiff base (Lagarias and Lagarias, 1989). In darkness, this autoassembly produces the redlight-absorbing form Pr (lmax ' 660 nm). In red light this is photoisomerized to another form, Pfr, which absorbs maximally in far-red light (lmax ' 730 nm). In far-red light, Pfr is in turn converted back to Pr. Both forms are thermodynamically stable and can be interconverted by any number of photocycles. Because even tiny amounts of Pfr have major physiological effects, it is generally accepted that this is the active form of phytochrome, while Pr seems to be physiologically inactive. In plants, phytochromes control a variety of developmental processes such as seed germination, stem elongation, construction of the photosynthetic apparatus, chloroplast movements, shade avoidance, and photoperiodic induction of flowering. In lower plants they are also involved in sensing light direction.