Components of Light-Induced Signal Transduction in Cyanobacteria

Abstract

Sensing the quality of light with respect to intensity, spectral composition, duration, and direction and polarization is of outstanding importance for photosynthetic organisms, being them immobilized as the plants or motile as many bacteria or unicellular algae ([Smith 2000]). This capability might also be of value for nonphotosynthetic parasitic bacteria searching and invading plants. Phytochromes, the ubiquitous plant photoreceptors, absorb light around 660nm, and are converted by light into the signaling state, absorbing around 730nm, called P_r (red-) and P_fr- (far red absorbing) forms ([Braslavsky et al 1997]). Plant phytochromes carry a covalently bound open-chain tetrapyrrole chromophore (phytochromobilin, PΦB, in only few cases phycocyanobilin (PCB) ([Jorissen et al 2002a],[Wu et al 1997]) that undergoes a photoisomerization at one of its double bonds. The covalent attachment of the chromophore to the protein is accomplished in all plant phytochromes via a thioether formed between the thiol group of a cysteine residue and the 3′-position of the ethylidene substituent at ring A of the bilin (Figure 1).