Cyanobacteria are the most primitive photosynthetic oxygen evolving prokaryotes that are ubiquitously found in nature [1]. They are thought to be the forerunners of plant chloroplasts [2]. The majority of Earth's atmospheric oxygen is produced by these microbes [3]. They can also fix atmospheric N2 and CO2 in large quantities thereby increasing the biomass in terrestrial as well as aquatic ecosystems [4]. Cyanobacteria can be used in waste-water treatment as they colonize those habitats containing high concentration of phosphate, nitrate, and heavy metals [5], [6]. Cyanobacteria are the source of a large number of economically important secondary metabolites having pharmaceutical, industrial, and ecological importance to humans [6]. These microorganisms have been cultivated and used as biofertilizer, food and biofuels with minimal growth requirements (solar light, water, CO2) with high productivity per unit area [6], [7], [8]. Nevertheless, cyanobacteria have been impacted by a variety of abiotic stressors, including pH, heavy metals, salinity, temperature, nutrition availability, and light quality and quantity. Being photoautotrophic in nature, cyanobacteria are constantly subjected to solar radiations that may affect the photosynthetic machinery, DNA, proteins, and lipids [9], [10]. Ultraviolet radiation (UVR) is a well-known mutagenic agent that inhibits the efficient expression and transfer of genetic material [11], [12]. UVR-mediated DNA damage such as pyrimidine-pyrimidone (6-4) photoproducts and cyclobutane pyrimidine dimers (CPDs) may cause mutagenesis and ultimately death of the living organisms if remains unrepaired [13], [14], [15]. Furthermore, many biotechnologically altered cyanobacteria seemed to be genetically vulnerable when it comes to implement DNA recombination to eliminate/incapacitated newly intercalated genes of economic significance [1]. In response to abiotic stresses, cyanobacteria have developed repair strategies such as photoreactivation to repair those lesions for cell survival and proper metabolic functioning. In the presence of blue light, cyanobacteria use an enzyme called photolyase to restore the normal state of CPDs formed by UVR [16], [17].

Photoreactivation is a novel mechanism found in cyanobacteria that repairs UV-induced DNA lesions (thymidine dimer or 6-4 photoproduct) to normal state with the help of enzyme called photolyases (Phrs). Phrs are photon energized nanomachines that harness the blue light photons and repairs thymine dimer [16], [17], [18]. It is present in all the three domains of life. Phrs have two light absorbing cofactors e.g., methyltetrahydrofolate (MTF) (folate) and FADH-. The MTHF serves as an antenna that absorb blue light photons (300-500 nm) and the exciting energy is channelled to FADH-. The enzymes form ionic bond with the phosphate residues of the damaged DNA strand and flip the thymine dimer dinucleotide into the active pocket such that the T< >T interacts with FADH via Vander Waals forces. The excited FADH- cleaved the cyclobutane ring via cyclic redox process to change T< >T to T-T and the repair DNA detached from the enzymes [16], [18], [19].

Computational analyses on cyanobacterial photolyase have identified five different groups of photolyases such as CPD Gr I, 6-4 Phrs/cryptochrome, Cry-DASH, Fe-S bacteria Phrs, and a group with less amino acid (276-385) in length [17]. Group V photolyase are solely present in cyanobacteria having Fe-S protein binding site. Group II and III photolyases had novel motif and DNA binding sites.

Pyrimidine dimers or DNA intra-stand cross linkages that causes the double helix of DNA to become distorted are repaired via the nucleotide excision repair (NER) process. In E. coli two proteins i.e., UvrA and UvrB form the complex UvrAB which recognises the DNA lesions. UvrC creates a 2-fold incision on both ends of the lesions. Thereafter, the single-strand DNA bearing the lesions is incised by UvrD. The missing DNA is synthesized by the DNA polymerase I (Pol I), and joined by the ligase [16], [19]. In cyanobacteria, NER proteins are present but detailed functions are not well explored. In order to comprehend the function of these proteins and their potential significance in DNA repair and recombination (which may be exploited to introduce novel proteins with economic relevance), a complete study of the cyanobacterial NER proteins is required. Therefore, we have studied the phylogenetic distribution and structure, interaction of T5-T6 decamer with the UvrAB complex and UvrB DNA repair proteins of cyanobacteria. Furthermore, the molecular docking of UvrA and UvrB NER DNA repair proteins was also analysed. The resulted complex UvrAB was used for further interaction study with the T5-T6 decamer.