## 1 Introduction  
Microorganisms respond to changing light conditions using an evolved repertoire of photoreceptors that perceive light and execute a light-dependent control of regulatory 'output' domains [1]. Blue-light using flavin (BLUF) protein photoreceptors respond to blue light and are often coupled with different effector domains (enzymes or transcriptional regulators) to generate the full range of combinations to regulate photo-adaptive responses [2; 3; 4; 5]. However, proteins having the BLUF domain with an extended C-terminus only have also been reported, and their responses were controlled by light-dependent protein-protein interactions [6; 7; 8; 9]. Upon illumination, the isoalloxazinemoiety of the BLUF domain associated flavin chromophore (flavin adenine dinucleotide; FAD, or flavin adenine mononucleotide; FMN, or riboflavin; RF) absorbs blue light [10], and undergoes structural rearrangements to modulate the communion between BLUF and the effector domains [11]. Unlike the complex mechanisms of photo-transformation in other photoreceptors, the BLUF domain, upon illumination, mainly shows a hydrogen bond rearrangement around the flavin cofactor, which causes a 10-15 nm red shift in the BLUF absorbance peak [11]. The photo-activation of the BLUF domain is due to the involvement of a conserved glutamine and tyrosine residues [12; 13; 14]. The hydrogen bond rearrangement involves a unique ability of the BLUF domain, i.e., photo-induced proton-coupled electron transfer (PCET), which enables them to switch between receptor and signaling states [15; 16; 17]. The photo-activation and structural rearrangements around the chromophore of the BLUF domain are transmitted as a signal for the activation of an associated effector domain. The BLUF domains are considered as an attractive model to investigate new paradigms of photo-induced signaling, BLUF domains have a modular architecture; hence, they may be functionally fused to different effector domains, as observed for the modular light oxygen voltage (LOV) photoreceptors [18; 19; 20; 21; 22; 23; 24; 25]. [PERSON] and coworkers have characterized a full-length active photoreceptor, BlrP1, from _Klebsiella pneumoniae_, which is composed of BLUF and EAL (Glutamine-Alanine-Leucine) as the sensor/output domain combination, respectively [4]. The EAL domain is a conserved signature motif which hydrolys cyclic dimeric GMP (c-di-GMP) and is involved in the regulation of motility, biofilm formation, virulence and antibiotic resistance in the bacteria [26; 27; 28; 29]. When exposed to light, the BLUF domain from BlrP1 activates the EAL domain via an allosteric communication relayed through conserved domain-domain interfaces [4]. In _Escherichia coli_, YcgF is another photoactivated protein with the BLUF-EAL domain that has been reported [30]. However, unlike other EAL domain proteins, YcgF acts as a transcriptional regulator and controls the YcgF/YcgE pathway, which regulates the synthesis of small regulatory proteins. These small regulatory proteins are involved in the modulation of biofilm functions via the Rcs two-component pathway, necessary for _E. coli_ to sustain the adverse environment [30]. In many bacteria, proteins with tandem GGDEF (diguanylate cyclase; DGC)/EAL (phosphodiesterase) domains were also reported to be involved in the c-di-GMP turnover, which modulates a variety of functions ranging from the functional modification of cell surface components, the expression of extracellular signaling molecules, virulence and motility [31; 32]. Photoactivated BLUF associated adenylyl cyclase homology domains (CHD) were also investigated and well characterized in several microorganisms, where they are specifically involved in the catalytic conversion of ATP to cyclic AMP (cAMP), which regulates the downstream signal transduction [33; 34; 35; 36; 37; 38]. PAS domains (Per/ARNT/Sim) are one of the broadly spread domains involved in sensing variations in light, oxygen, redox potential and the binding of small ligands [39].