## 4 Conclusions  
Applications of the photoreceptors in order to quickly control molecular machines and, in turn, biological systems and processes, present the scientific community with an exciting opportunity with several possibilities, approaches, along with their limitations as well. Recently, several such approaches have been reported, including photoswitches, UV photo-reactivation and deactivation, spectral tuning, and photocaging [25]. However, a detailed understanding of electron transfer mechanisms, transient state intermediates, and amino acid patterns will allow the development of more precise recombinant techniques, fusion proteins and complexes for improving such systems and providing an alternate route to design broadly reactive light-sensitive probes. In the case of LOV domains, the deprotonation of flavin N (5) involves rate-determination for the recovery, using base catalysis, pH, proton inventory and structural studies [173]. It will also be interesting to study mechanisms to convert these small spectral shifts into more significant jumps leading to many fold increases in the rage of 400-500 nm in the activities of such receptors and downstream signals. Theoretically, the fold increase in the signal response could be improved to that extent [173; 174]. Biocatalytic reactions using photoactivated enzymes, produced either through recombinant methods or through directed evolution, can reduce the complexity of the system by controlling the system remotely to deliver high-value materials and compounds in biotechnology or the pharma industry. Photo-controlled receptors like BLUF can play a vital role in biotransformation cascade in the same way as that of photosensitive chemical groups like O-nitrobenzyl, 3-nitrophenyl and benzyloxycarbonylphenylinic organic reaction steps, where the reaction's product acts as the substrate for the next reaction in a multistep pathway [175]. We have identified and analysed 34 such proteins containing the BLUF domain in association with different effector domains, such as kinases, phosphatases, phosphodiesterase, a transcriptional repressor (TetR), PAS, endonuclease, PBP proteins, etc., involved in regulating a wide range of cellular processes. All of the selected proteins have a conserved catalytic core, including tyrosine, glutamine, and tryptophan or methionine, which are essential for the BLUF photo-activation and photocycle. Until now, several photoreceptors, such as channelrhodopsin2 (ChR2) [176], phytochrome [177], cryptochromes [178], LOV [19], as well as BLUF [4; 30], have been adopted for their optogenetic potentials. However, considering the modularities in the BLUF domain architecture and their unexplored nature, these combinations have a great potential to befurther utilized for the development of novel optogenetic technologies. To conclude, these photoactivated/controlled systems could be the way forward in synthetic biology, as different and subtle differences in the light sensitivities of these vast arrays of receptors can be harnessed to regulate a reaction cascade in an engineered organism by choosing a particular photoreactivity and control. **Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/9/18/3924/s1](http://www.mdpi.com/2076-3417/9/18/3924/s1)]([http://www.mdpi.com/2076-3417/9/18/3924/s1](http://www.mdpi.com/2076-3417/9/18/3924/s1)), Figure S1: Multiple sequence alignment of the BLUF coupled EAL domain. Sequence representing the EAL domain from the BlrP1 protein was used as the template for the sequence alignment analysis. Amino acid residues in solid box are the conserved residues involved in the formation of the EAL active site, Figure S2: Multiple sequence alignment of the BLUF coupled CHD domain. Sequence representing the CHD domain from the bPAC protein was used as template for the sequence alignment analysis. Amino acid residues in solid box are the conserved residues involved in the formation of the nucleotide binding site, Figure S3: Multiple sequence alignment of the BLUF coupled PAS domain.