**5. Color-tuning mechanisms of cyanobacteriochromes**

The term cyanobacteriochrome was first reported in 2004 by Dr. Ikeuchi in his paper about photoreceptor SypixJ1 [60]. This photoreceptor covalently binds a linear tetrapyrrole chromophore and performed reversible photoconversion between a blue-absorbing form (Pb) and a green-absorbing form (Pg). This protein has a cGMP-phosphodiesterase/adenylate cyclase/FhlA (GAF) similar to those of the Phytochromes. The GAF in cyanobacterial signals transduction proteins is identified as CBCRs. The GAF s of cyanobacteria are covalently linked to tetrapyrrole chromophore. This chromophore can sense different light (UV–visible spectra). In some cyanobacteria, this GAF regulate phototactic motility, chromatic acclimation and light-dependent cell aggregation.

### **5.1 CBCR structure and diversity**

In CBCRs, the GAF is essential for chromophore incorporation and photoconversion [61] CBCRs have N and C terminals in which GAF s are located at the N-terminus. Signal output s viz. (HisKA + HATPase\_c), Methyl-accepting (MA), GGDEF, and EAL s located at the C- terminus. His kinase is frequently detected as signal output s.

### **5.2 Chromophore variation in color-tuning mechanism**

Generally, four types of linear tetrapyrrole chromophores, phycocyanobilin (PCB), biliverdin (BV), phytochromobilin (PFB) and phycoviolobilin (PVB), have been identified in the CBCR GAF domain. The mixture of these chromophores in the GAF domain results in broad and diverse spectral features [54, 62]. These chromophores are arranged in an order from a longer wavelength absorbing system to a shorter wavelength absorber (BV > PFB > PCB > PVB). The longer wavelength of light is absorbed by the longer length chromophores. Sometimes, the PCB isomerizes to PVB at the GAF region. This sort of PVB linked GAF area in CBCR has been distinguished from the cyanobacterium *Acaryochloris marina* [54]. The chromophore binding species should possess a UV-to-blue absorber.

The absorption of the Cys-free form is highly affected by the linked species of chromophores. The chromophore PCB in AM1\_1186g2 revealed a reversible photoconversion between a Cys-free red-absorber (Pr) and a Cys-dependent blue absorber Pb [63], while covalently linked PVB in TePixJg showed reversible photoconversion between a Cys-free Pg absorber to a Cys-linked Pb absorber [62, 64]. The Cys-linked Pb absorber in the CBCR GAF s is the same as the blue to green reversible, but the Cys-free teal-absorber (Pt) is often shifted to blue absorber in association with

*Cyanobacterial Phytochromes in Optogenetics DOI: http://dx.doi.org/10.5772/intechopen.97522*

the typical green absorber Pg. A twist in the D ring of the conserved Phe residues in the Pt absorber contributes to the blue-shift [65, 66]. The conversion of the Pt form into the typical red-shifted Pg form is mediated by the loss of these Phe residues [66]. Likewise in the XRG lineage, CBCR GAF s have red to green reversible photoconversion. Blue-shift of the Pg form to red absorbing Pr caused a small twist in the A and D ring [67]. This photocycle is mediated by the proton donor and the acceptor is Glu amino acid.

### **5.3 Dark reversion**

The two distinctive light-harvesting types of the CBCR GAF domain are generally constant under the dark phase, thus these CBCR GAFs can detect the proportion of two wavelengths. Further, in some CBCR GAF showed unidirectional photoconversion and rapid dark reversion. This can identify the concentration of certain colors of light [68]. Some XRG CBCR GAF s are unidirectional photoconversion from Pr-to-Pg in dark conditions, and after 4–25 s it rapidly undergoes dark reversion from Pg-to-Pr [68]. The kinetics of these GAF s of the dark reversion is highly dependent on higher temperature [53, 69, 70]. Light and temperature were indulged in the regulation of these GAFs. These characteristics may be physiologically relevant to sense light intensity for efficient photosynthesis because the same light intensity with lower temperatures severely inhibited photosynthesis.

### **5.4 Engineering**

Several CBCRs have been designed to change the color-tuning interaction and output activity. Inclusion of the second Cys residue and modifications of PCB-binding in the GAF s caused reversible photoconversion from red/green into blue/green. This is due to the isomerization of PCB to PVB by the incorporation of the second Cys residue which attaches to the chromophore in the reversible form [70, 71]. The twisted geometry of the D ring can also be removed [66]. The output activity of the native GAF was modified with other lineage s by adenylate cyclases [72–74] that respond to various light. Changing the length of the CBCR GAF linker region in CcaS and HisKA turns the light receptiveness of the green to red lineage [75].
