**3.1 Coordination of the cyanobacteriochromes**

The photo biochemical properties of SesA holoprotein from the cyanobacterium *Thermosynechococcus vulcanus* have a blue light-responsive DGC (Diguanylate cyclase) activity. The SesB holoprotein isolated from *T. vulcanus* exhibited a reversible photoconversion system. It becomes blue light (417 nm) capturing form to a teal light (498 nm) assimilator. Another homologous CBCR from *T. vulcanus* is SesC which photoconverts a blue light (415 nm) assimilator to a green light (522 nm) absorber. These three CBCR proteins (SesA, SesB, and SesC) have phycoviobilin (PVB) and phycocyanobilin (PCB). These CBCR proteins were genetically expressed in *E. coli* which contains both PVB and phycocyanobilin [22, 23]. The SesA and SesB, perform independent photo conversion in *E. coli* in contrast when it is expressed in cyanobacteria it shows single photoconversion (**Figure 2**). Even though their spectral wavelength is different they coordinate and expressed single photocycle conversion.

SesB has GGDEF- type DGC (Diguanylate cyclase) domain (**Figure 3B**) and SesC has EAL- type PDE domain to deliver the c-di-GMP signal (**Figure 3C**). The SesB

### **Figure 2.**

*Three different CBCR individually expressed to reveal different c-di-GMP signals. Ses A-produces c-di-GMP under blue light, Ses B- degrades c-di-GMP under teal light, Ses C- produces c-di-GMP under shorter wavelength and degraded c-di-GMP at the longer wavelength. These CBCR were coexpresses in Thermosynechococcus revealed c-di-GMP signal binds with cellulose synthase domain and promoted cell aggregation.*

### **Figure 3.**

*Domain architecture of cyanobacteriochromes Ses A, Ses B and Ses C-GAF photosensitive domain and cell signaling domain PAS (Per/Arnt/Sim), GGDEF, EAL capped with a.a-amino acids.*

DGC for c-di-GMP signal degraded under teal light, in contrast, expressed higher in blue light. In Ses, A c-di-GMP is higher under blue light and lowered in teal blue light. SesC DGF activity is maximum in blue light and minimum in green light. This is a chrome-responsive cyanobacterial c-di-GMP signaling coordination of (Ses –A, B and C) CBCRs.


## **3.2 Cyanobacteriochrome in cell aggregation**

The cell aggregation signaling molecule Cyclic dimeric guanosine monophosphate (c-di-GMP) is unique to cyanobacteria and bacteria [24]. Light is a key factor in controlling c-di-GMP signaling [25, 26]. The domain (GGDEF) for the synthesis and (EAL/HD-GYP) (**Figure 3A** and **B**) destruction of the c-di-GMP is higher in the CBCR GAF structure of freshwater cyanobacterial genomes. The CBCR induces the c-di-GMP signaling pathway. The CBCR—GAF domain of SesA (**Figure 3A**) from the thermophilic cyanobacterium *Thermosynechococcus elongatus* is activated by blue light irradiation, and disordering of *T. vulcanus* SesA inhibited cell aggregation.

*Thermosyncechococcus* spp., genomes possess five CBCR genes, three homologous CBCRs involved in the clumping of cyanobacterial cells are SesA (Tlr0924), SesB (Tlr1999), and SesC (Tlrtml). This CBCR has a photosensory domain with a c-di-GMP protein production/destruction domain. The CBCR-GAF domain of these three CBCR is involved in the light-controlled cell accumulation. There is a coordinated system of cell accumulation by c-di-GMP signaling via, Ses (A, B and C) CBCR (**Figure 2**) [27, 28].

## **3.3 Cyanobacterial photobiological responses**

Prokaryotic photosynthetic organisms, cyanobacteria, depend on bilin-linked phytochromes (Cphs) and CBCRs, photoreceptors which are structurally and functionally vary from plant photoreceptors. The CBCRs are made of light-absorbing domains with various color-tuning and signal transmission processes, that make cyanobacteria capture a wide wavelength of light from UV–visible to far-red lights. The genome of filamentous cyanobacteria has a different type of CBCRs with wide chromophore-linked selectivity and photocycle protochromicity. The Cph lineage can absorb a wide range of light from blue-violet to yellow-orange light. This chapter also emphasized the color-sensitive diversity [29, 30] and signal transmission process of Cphs and CBCRs, concerning optogenetic.

Bilin-linked phytochrome Cphs and plant phytochromes (Phys) are similar in structure, with an N-terminal photosensory core module (PCM) and a C-terminal output regulatory module. The PCM contains the following domains PAS (Period/ Arnt/Single-minded), GAF(C-GMP phosphodiesterase/Adenylylcyclase/FhlA), and PHY (phytochrome-specific). The GAF domain is necessary for forming the bilin cross-linking; PAS and PHY structures are involved in bilin lyase activity [31]. Cyanobacteria have two types of bilin-linked photoreceptors Cphs, and CBCRs. In contrast to Cphs with PAS and PHY domain, CBCRs (lack PAS and PHY) absorb a wide array of light, by the GAF structure [32]. This wide array of light absorption by CBCR is called a color or spectral tuning mechanism.

### **3.4 CBCR in photobiological responses**

Growth of the cyanobacterium *Synechocystis* PCC 6803 in red (R) and far red (FR) light is regulated by Cph1 and Cph2 in an antagonistic method. Modification in Cph1 negatively affects the *Synechocystis* growth in FR light, further destruction of Cph2 hinders its growth in red light [33]. Mutation in Cph2 transformed the growth rate and exopolysaccharide biofilm formation, involved in the control of the principal energy metabolism [34]. Under unusual light environments, the bilin conformation of the cyanobacterial antenna with light-absorbing phycobilisomes rearrangement is known as chromatic acclimation (CA). This process allows cyanobacteria to neutralize the proportion of light absorption between the photosystems [35, 36].

## **3.5 Dual light system**

The CBCR response to two different light systems is mediated by the histidine (His) kinase domain. In *Leptolyngbya* sp. JSC-1, His domain is found in the proteins of Cph, RfpA, whereas CcaS in *Synechocystis* and *Nostoc punctiforme*, RcaE and DpxA in *Fremyella diplosiphon*, act as sensor kinase [35].
