**1. Introduction**

Photoreceptors in cyanobacteria are diverse in their spectral character from ultraviolet to visible wavelength. Plant photoreceptors were widely used in optogenetics, but their responses to specific wavelengths need more revision. When compared to these photoreceptors cyanobacteriochromes (CBCRs) receive more attention as a versatile optogenetic tool. Several photoreceptors respond to a wide range of light, photoconversion ability and photoswitches for dual light are new approaches and powerful tools for optogenetics [1]. Engineering of these photoreceptors will develop more versatile CBCR to alleviate the conventional methods like mutation and recombination [2]. Optogenetics in mammalian tissue adopted far-red illumination and adjacent infra-red radiance to visualize and activate responses in the cell. The CBCRs with linear tetrapyrrole is very sensitive to red and far-red light. Utilization of these infra-red sensitive and red light responsive CBCRs raised their application in optogenetics. So far phytochromobilin is used in mammalian cells recently cyanobacterial phytochrome 1 (CPH1) has been applied in mammalian cells proven its benefit in synthetic biology [3].

### **1.1 Cyanobacteria**

Cyanobacteria are evolutionarily ancient phototrophic Gram-negative bacteria widely distributed in terrestrial, freshwater and marine environments. They are oxygenic photosynthesizers having major photosynthetic pigment chlorophyll-a and light-harvesting pigments phycobiliproteins. They survive in many extreme environments, such as hot and cold deserts, hot springs, and hypersaline environments [4].

### **1.2 Cyanobacteriochrome**

Light is an important factor for their nutrition and growth, therefore, it has a multitude photosensory complex that responds to a wide array of illumination. Each chromophore is a response to a particular wavelength based on the incident light it changes the arrangement and composition of pigments in the photon capturing antenna. This rearrangement of pigments to the incident light is the process of complementary chromatic acclimation. Cyanobacteria possess phototaxis movements it means they can move towards or away from specific light. Photoreceptors in cyanobacteria are commonly referred to as CBCRs [5].

### **1.3 Phytochromes**

Generally, Phytochromes are photoreceptors that have been found in plants, algae, and bacteria. These photoreceptors are broadly utilized in biosensors and optogenetics to screen and regulate diverse intracellular cycles like phosphorylation, gene activation, degradation of protein and change of calcium ions [6].

### **1.4 Phytochromes from cyanobacteria**

Phytochromes are photochromic photoreceptors, generally responding to red and far-red radiation in the visible spectrum. Bilin is the most important portion in the chromophore and it is distributed in three different forms. Phytochrome in plants made of phytochromobilin, whereas in cyanobacteria it is in the form of Phycocyanobilin. Further Phytochromes in plants, algae and cyanobacteria constitute linear tetrapyrrole biliverdin [7]. The chromophore part in plant phytochrome has cysteine at the N terminal site of the protein. The phytochrome in plants differs from cyanobacteria by having biliverdin in the chromophore part. Evolutionary development in cyanobacteria brings out cysteine linked with biliverdin in the GAF domain and formed as phycocyanobilin also referred to as phytochromobilin. The transformation of phytochrome into CBCR is due to changes in the molecular level.

### **1.5 Phytochrome classification**

Phytochromes were primarily arranged into three subfamilies dependent on the number of domains in their photosensory core module (PCM). Phytochrome has

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

three domains in their core structure, for example, PHY - phytochrome-explicit area, PAS - Per-Arnt-Sim, and GAF - cGMP phosphodiesterase-adenylate cyclase-FhlA. Even though the amino acid groupings of these domains have a dissimilar sequence, their structures were similar. Further subfamilies are cyanobacterial phytochromes (Cph), lack an N-terminal PAS area, and CBCRs, which contain a solitary GAF domain [8]. The domain proteins of PAS, GAF and PHY were interconnected to form homo and heterodimers [9].

### **1.6 Features of cyanobacteriochrome**

Phytochrome in plants and algae has the sensitivity to the different light spectrum. Plant phytochromes are sensitive to red radiance furthermore, it performs red and far-red photoreversible photocycle. The phytochrome with bilin photoreceptors in eukaryotic green algae and prokaryotic cyanobacteria are sensitive to the visible spectrum [10–12].

The CBCRs are photoreceptors involved in the regulation of phototaxis. The photoreceptors SyCcaS, SyPixJ1, TePixJ, AnPixJ, SyCikA are now proposed to be CBCRs due to the presence of chromophore binding GAF domain.

