Phytochromes: Light Triggered Molecular Switches

Tilman Lamparter

Institut für Pflanzenphysiologie und Mikrobiologie, Fachbereich Biologie, Freie Universität Berlin, Königin-Luise-Str. 12-16, D-14195 Berlin, Germany

e-mail: lamparte@zedat.fu-berlin.de
 

Phytochromes are photochromic photoreceptors that modify a broad spectrum of developmental processes in plants, various procaryotes and slime molds. Biosynthesis occurs in the Pr-form, which has an absorbance maximum around 660 nm. Red irradiation yields the thermostable Pfr, which absorbs around 730 nm. Pfr is converted back to Pr by long-wavelength red light. Phytochromes can be used as light-triggered switch: the Pr/Pfr ratio as given by the last light treatment is stored during the subsequent dark period. Biochemical studies on plant phytochromes started > 40 years ago with its spectral detection in tissue and aqueous extracts. Plant phytochromes are homodimers, the subunit a ca. 120 kD protein with a covalently attached bilin chromophore. Chromophores are incorporated autocatalytically in vitro, this allows free choice of natural and synthetic bilins if recombinant phytochromes or chromophore-deficient mutants are used. Photoconversion from Pr into Pfr via different spectrally and kinetically characterized intermediates is completed within ca. 2 s. During this photoconversion, the chromophore undergoes an isomerization around the C15=C16 double bond. This is accompanied by conformational changes of the protein that are monitored by e.g. chromatography, limited proteolysis or chemical modification. Light-dependent serine / threonine kinase activity has been shown for recombinant plant phytochromes, in addition, a number of phytochrome-interacting proteins are being discovered. The mode of action and signal transduction is however not completely understood.

Prokaryotic phytochromes have been discovered only recently [1]. They are of smaller molecular size than plant phytochromes, and belong to the group of two component histidine kinases. Typically, histidine-autophosphorylation and transfer of the phosphate group to a receiver protein are dependent on the conformation of phytochrome. The cyanobacterial phytochrome Cph1 can be expressed at high levels in E. coli. Conformational changes during photoconversion were monitored with various biophysical techniques, pointing to similarities and differences to plant phytochromes.

The characteristics of phytochrome offer a number of technical applications in the future. (i) When assembled with phycoerythrobilin, phytochromes display a typical fluorescence with high quantum yield. Thus phytochromes can be used as molecular marker, like GFP [2]. Mutagenesis might yield a phytochrome that fluoresces with the natural chromophores phytochromobilin/phycocyanobilin. (ii) Genes for phytochrome, chromophore biosynthesis and signal transduction elements can be transformed into E. coli for light-controlled expression of recombinant proteins. (iii) If the possibility is given to read out the state of phytochrome by e.g. electronic devices, phytochrome can be used as biochemical light-triggered memory.

Literature

[1] J. Hughes and T. Lamparter, Plant Physiology 121, 1059-1068 (1999).

[2] J.T. Murphy and J.C. Lagarias, Curr. Biol. 1, 870-876 (1997).