**2. CBCR in cyanobacteria**

### **2.1 AnPixJ**

The cyanobacterial genomes of Anabaena and Nostoc harbor *PixJ* homologs, having chromophore-linked GAF domains and domain MCP. The PixJ-GAF domains of *Anabaena* and *Nostoc* were distinct from the blue-shifted complex of CBCR TePixJ and CBCR SyPixJ1 [13]. The four GAF domains of PixJ are continuously arranged in AnPixJ of *Anabaena* sp. PCC 7120 (**Figure 1A**) that possess reversible photoconversion between red (648 nm) Pr[AnPixJ] to green (543 nm) absorbing form Pg (AnPixJ) [14]. Acidic denaturation of AnPixJ in *Anabaena* sp. PCC 7120 affected the gliding motility of hormogonia and phototaxis.

## **2.2 SyCcaS**

Chromatic acclimation is an adaptive mechanism in some cyanobacteria capable of modifying their photosynthetic system reaction to the incident radiance. The phycocyanin content in *Synechocystis* sp. PCC 6803 is chromatically synchronized under red and green-orange light. The cells irradiated with red light produced a higher quantity of phycocyanin [15] than the cells exposed to green-orange light. The red

**Figure 1.**

*Domain architecture of common cyanobacteriochromes (A) AnPixJ (B) SyCcaS (C) FdRcaE (D) SyCikA and their specific domains GAF with additional signaling domains are HAMP, methyl accepting chemotaxis protein (MA), PAS (PAS+ PAC- Photoswitchable adenyl cyclase), histidine kinase (HisKA+HATPase) and response regulator receiver domains (REC).*

light condition activated the gene cpcG2 which encodes the synthesis of phycocyanin linker protein. Under red light CcaS, photoreceptor and transcriptional regulator CcaR induced the expression of the *cpcG2* gene [16]. It has a single GAF domain followed by PAS and PAC domains (**Figure 1B**).
