## 3 Results and Discussion
### BLUF Modules in Association with the Effector Domains
#### 3.3.5 B\({}_{12}\) Binding Domain  
Many prokaryotes synthesize vitamin B\({}_{12}\) (cobalamine) having a tetrapyrrole-like structure composed of a bound cobalt atom (Co) with two axial ligands. The lower ligand is known to be involved in vitamin B\({}_{12}\) binding, while the upper one is used as a cofactor for different groups of enzymes/proteins, such as methyltransferases, reductases and isomerases [92]. The vitamin B\({}_{12}\) binds to a specific domain, i.e., Asp/Glu-X-His-X-X-Gly-(41)-Ser/Thr-X-Leu-(26-28)-Gly-Gly, which is highly conserved in almost all B\({}_{12}\)-dependent enzymes/proteins and characterized as a Rossmann fold, typically composed of 5 parallel \(\beta\)-sheets surrounded by 4-5 \(\alpha\) helices [93; 94]. Generally, vitamin B\({}_{12}\) and its derivatives are known for their role in fatty acid and folate metabolism; however, recently they have been characterized as photoreceptors with a novel and unanticipated biological function as a light dependent transcriptional regulator [95]. [PERSON] et al. [96] reported, for the first time, a light-induced excitation of CarH (Mer-like transcriptional factor/repressor) bound adenosylcolabalamin (AdoB\({}_{12}\)), which inhibited the formation of a stable CarH-AdenoB\({}_{12}\) tetramer, thus allowing the gene expression for the carotenoid biosynthesis in _Myxobacteria_. In the dark, the stable CarH-AdoB\({}_{12}\) tetramer binds at the promoter region and shuts down the carotenoid biosynthesis [95; 96; 97]. Upon light illumination of the corrin ring associated with AdoB\({}_{12}\), it promotes the re-orientation of a helix bundle forming a covalent linkage between H132 and Co, and causes the CarH dissociation from the promoter region, which ultimately leads to the carotenoid gene expression [95; 96; 98]. [PERSON] et al. [92] also reported a small stand-alone B\({}_{12}\)-binding domain protein, AerR in _Rhodobacter capsulatus_, which controls the light-dependent regulation of the biosynthesis of the photosystem via interacting with Crtl, a repressor of the photosystem gene expression. Like CarH, the light illumination also leads to a covalent association between the His10 and Co ligand, which suggested that the light-dependent covalent linkage between the Co ligand and His residue might be the common mechanism in B\({}_{12}\)-dependent photoreceptors. Furthermore, photoreceptors have also been naturally endowed with multiple photosensory domains, which could relay signals to output domains to control specific light-dependent functions [95]. In the present communication, we have selected a modular protein (accession number ABP71929.1) with the BLUF and vitamin B12 binding domain from _Rhodobacter sphaeroides_ ATCC 17025 (Table 1). We performed the alignment of the selected protein with the well-characterized B\({}_{12}\) binding domain containing proteins (CarH and AerR) and, surprisingly, observed that the crucial amino acids (Trp131, Val138, Glu141 and His142) involved in forming the binding pocket for AdoB\({}_{12}\)[95; 99] were not observed in the selected protein sequence (Figure S4). Moreover, there are many prokaryotes which do not necessarily require the B\({}_{12}\) cofactor; hence, they can acquire alternative B\({}_{12}\)-independent metabolic pathways for the same reaction [100]. The association of the B\({}_{12}\) domains with the BLUF domains was also reported by [PERSON] et al. [92], where the role of the B\({}_{12}\) domain is to sense light that is out of the absorption range of flavin. In several proteins, B12 binding domains are also found in combination with heme/oxygen sensing globin domains; however, none of these proteins have been characterized [92]. In proteins with combinations of histidine kinases or serine/threonine kinases with the B\({}_{12}\) domain, the kinases are responsible for regulating the response to light absorption by the B\({}_{12}\) domain [93].