A Brief History of Phytochromes

Abstract

Photosensory proteins enable living things to detect the quantity and quality of their light environment and to transduce that physical signal into biochemical outputs which entrain their metabolism with the ambient light environment. Phytochromes, which photoconvert between red-absorbing Pr and far-red-absorbing Pfr states, have been the most extensively studied of these interesting proteins. Critical regulators of a number of key adaptive processes in higher plants, including photomorphogenesis and shade avoidance, phytochromes are widespread in photosynthetic and nonphotosynthetic bacteria and even in fungi. Cyanobacterial genomes also possess a plethora of more distant relatives of phytochromes known as cyanobacteriochromes (CBCRs). Biochemical characterization of representative CBCRs has demonstrated that this class of photosensors exhibit a broad range of wavelength sensitivities, spanning the entire visible spectrum. Distinct protein-bilin interactions are responsible for this astonishing array of wavelength sensitivities. Despite this spectral diversity, all members of the extended family of phytochrome photosensors appear to share a common photochemical mechanism for light sensing: photoisomerization of the 15/16 double bond of the bilin chromophore.

Green/red cyanobacteriochromes regulate complementary chromatic acclimation via a protochromic photocycle

“…Similarly, it has been suggested that two-Cys CBCRs can sense the cellular redox potential via oxidation of the second Cys, which arrests the photocycle (21). There also are many examples of tandem CBCR sensors containing multiple GAF domains that perceive light of various colors (1,2,14,19,23,24). CBCRs thus are exceptionally sophisticated photosensors, able to integrate light with various physiological cues and to provide complete coverage of the entire visible spectrum with a single chromophore precursor and a small, soluble protein sensor.…”

“…S1B). * Cyanobacteriochromes (CBCRs) are widespread cyanobacterial photosensors with phytochrome-related GAF domains (1,2,13,14). Although CBCRs also convert between two photostates via bilin photoisomerization at C15, they exhibit much more spectral diversity, with peak absorptions ranging from 330 to 680 nm and hence spanning the entire visible spectrum and near UV (13,(15)(16)(17)(18)(19)(20)(21)(22)(23)(24).…”

“…light sensing | phycobiliprotein | signal transduction | spectral tuning | two-component signaling P hytochrome photosensors initially were discovered in plants and later found in cyanobacteria, nonoxygenic photosynthetic bacteria, nonphotosynthetic bacteria, fungi, and algae (1,2). These photoreceptors bind linear tetrapyrrole (bilin) chromophores within a conserved GAF (cGMP phosphodiesterase/adenylyl cyclase/FhlA) domain via a covalent thioether linkage to a conserved Cys residue ( Fig.…”

Structure of the Cyanobacterial Phytochrome 2 Photosensor Implies a Tryptophan Switch for Phytochrome Signaling

“…(SynCph2) (10) and several other cyanobacterial phytochromes (11). Finally, the cyanobacteriochromes (CBCRs, Group III) utilize a single, bilin-binding GAF domain (11,12). CBCRs cover a broad range of the visible/near UV spectrum but are restricted to cyanobacteria.…”

Distinct phytochrome actions in nonvascular plants revealed by targeted inactivation of phytobilin biosynthesis

“…Protonemata and gametophores of Physcomitrella patens subsp. patens were grown on solid Knop's medium or cultured in liquid PPNH 4 . Spores were germinated on solid Knop's medium supplemented with 10 mM CaCl 2 and overlaid with cellophane.…”