Cyanobacteriochromes are a newly recognized group of photoreceptors that are distinct relatives of phytochromes but are found only in cyanobacteria. A putative cyanobacteriochrome, CcaS, is known to chromatically regulate the expression of the phycobilisome linker gene (cpcG2) in Synechocystis sp. PCC 6803. In this study, we isolated the chromophore-binding domain of CcaS from Synechocystis as well as from phycocyanobilin-producing Escherichia coli. Both preparations showed the same reversible photoconversion between a green-absorbing form (Pg, max ‫؍‬ 535 nm) and a red-absorbing form (Pr, max ‫؍‬ 672 nm). Mass spectrometry and denaturation analyses suggested that Pg and Pr bind phycocyanobilin in a double-bond configuration of C15-Z and C15-E, respectively. Autophosphorylation activity of the histidine kinase domain in nearly full-length CcaS was up-regulated by preirradiation with green light. Similarly, phosphotransfer to the cognate response regulator, CcaR, was higher in Pr than in Pg. From these results, we conclude that CcaS phosphorylates CcaR under green light and induces expression of cpcG2, leading to accumulation of CpcG2-phycobilisome as a chromatic acclimation system. CcaS is the first recognized green light receptor in the expanded phytochrome superfamily, which includes phytochromes and cyanobacteriochromes.chromatic adaptation ͉ phycocyanobilin ͉ phytochrome ͉ cyanobacteria ͉ photoreceptor P hytochromes (Phys) are photoreceptors that typically perceive red and far-red light and regulate a wide range of physiological responses in plants, bacteria, cyanobacteria, and fungi (1). They exhibit reversible photoconversion between two distinct forms: the red-absorbing form (Pr) and the far-redabsorbing form (Pfr). Their N-terminal photosensory region, which consists of Per-ARNT-Sim (PAS), cGMP phosphodiesterase/adenylyl cyclase/FhlA (GAF), and phytochrome domains, is highly conserved, but there are variations in the chromophore of the linear tetrapyrrole, such as phytochromobilin, phycocyanobilin (PCB), and biliverdin. It is reported that phytochromobilin or PCB is covalently anchored at a conserved cysteine residue in the GAF domain (2, 3), whereas biliverdin is anchored at another conserved cysteine residue in the N terminus of the PAS domain (4). The perception of light by Phys triggers a Z to E isomerization of the C15-C16 double bond between the C and D pyrrole rings as well as subsequent conformational changes of the chromophore and the apoprotein [supporting information (SI) Fig. S1] (5) which signal to downstream processes. Recent crystallographic analyses of bacterial Phys (DrBphP and RpBphP3) have revealed the three-dimensional structure of PAS and GAF domains in the Pr form (6-8). The biliverdin chromophore is buried deep within a pocket in the GAF domain with a configuration of C5-Z,syn/C10-Z,syn/C15-Z,anti. Because the residues in the chromophore-binding pocket are highly conserved, it was proposed that Phys share a common photoconversion mechanism, albeit with certain variations.''Cyanobac...

The circadian oscillator of the cyanobacterium Synechococcus elongatus, like those in eukaryotes, is entrained by environmental cues. Inactivation of the gene cikA (circadian input kinase) shortens the circadian period of gene expression rhythms in S. elongatus by approximately 2 hours, changes the phasing of a subset of rhythms, and nearly abolishes resetting of phase by a pulse of darkness. The CikA protein sequence reveals that it is a divergent bacteriophytochrome with characteristic histidine protein kinase motifs and a cryptic response regulator motif. CikA is likely a key component of a pathway that provides environmental input to the circadian oscillator in S. elongatus.

;The gene, pixJ1 (formerly pisJ1), is predicted to encode a phytochrome-like photoreceptor that is essential for positive phototaxis in the unicellular cyanobacterium

BLUF (a sensor of Blue-Light Using FAD) is a novel putative photoreceptor domain that is found in many bacteria and some eukaryotic algae. As found on genome analysis, certain cyanobacteria have BLUF proteins with a short C-terminal extension. As typical examples, Tll0078 from thermophilic Thermosynechococcus elongatus BP-1 and Slr1694 from mesophilic Synechocystis sp. PCC 6803 were comparatively studied. FAD of both proteins was hardly reduced by exogenous reductants or mediators except methylviologen but showed a typical spectral shift to a longer wavelength upon excitation with blue light. In particular, freshly prepared Tll0078 protein showed slow but reversible aggregation, indicative of light-induced conformational changes in the protein structure. Tll0078 is far more stable as to heat treatment than Slr1694, as judged from flavin fluorescence. The slr1694-disruptant showed phototactic motility away from the light source (negative phototaxis), while the wild type Synechocystis showed positive phototaxis toward the source. Yeast two-hybrid screening with slr1694 showed self-interaction of Slr1694 (PixD) with itself and interaction with a novel PatA-like response regulator, Slr1693 (PixE). These results were discussed in relation to the signaling mechanism of the "short" BLUF proteins in the regulation of cyanobacterial phototaxis.