LIGHT REGULATION OF PHYCOBILISOME BIOSYNTHESIS AND CONTROL BY A PHYTOCHROME‐LIKE PHOTORECEPTOR

Ambient light quality changes dramatically affect the composition of light harvesting structures, the phycobilisomes, in many cyanobacterial species. In the cyanobacterium Fremyella diplosiphon , shifts in the ratio of red to green light lead to transcriptional changes and altered synthesis of several phycobilisome components. This process is called complementary chromatic adaptation (CCA). These two colors have opposite effects: red light activates an operon encoding the biliprotein phycocyanin (PC) and inactivates the operon encoding phycoerythrin (PE), whereas green light activates PE synthesis and shuts down PC synthesis. The effects of red and green light on CCA are photoreversible. Thus, CCA is similar to transcriptional processes that are controlled by phytochromes, a family of eukaryotic red/far red photoreversible photoreceptors. We are using molecular genetics to determine the mechanisms by which F. diplosiphon senses changes in the color of light of its environment. Initial mutant generation and complementation lead to the discovery of three CCA regulatory components that are part of a complex two component system. The most interesting of these is RcaE (regulator of chromatic adaptation), a histidine kinase‐class protein containing a region in its amino‐terminal half with similarity to the chromophore binding domains of phytochromes. Within this region, RcaE contains a cysteine residue in a similar location as that used for covalent attachment of the open‐chain tetrapyrrole chromophore in phytochromes. We will present recent data characterizing RcaE, including in vivo analysis of the chromophore that is attached to RcaE, as well as results from our recent isolation of a new CCA regulatory component.