## 3 Results and Discussion
### Modular Diversity of BLUF Domains
Sequence alignment studies of 34 BLUF domain containing sequences, in this study and elsewhere, show the conservation of tyrosine and glutamine residues in the \(\beta\)1 and \(\beta\)3 strands of the BLUF fold (Figure 4), and this has been reported to be critical for the photochemical reaction, as mutations in these amino acids result in the loss of its ability to perceive light [12; 13; 14; 17; 58]. In our sequence database, BLUF photo domains are fused at the N-terminus of a wide array of different effector domain modules, viz. kinases, phosphatases, phosphodiesterase, anti-sigma factors, DNA binding domains, a transcriptional repressor (TetR), PAS, endonuclease and PBP proteins. Some of these have been modeled with \(>\)90% confidence using the Phyre2 modeling program and are presented in Figure 4. These domains are part of various photoreceptors [1; 59; 60]. Our structural and field mapping analysis also shows that sequences like TetR_C_6, SRPBCC1 superfamily, GGDEF1 COG5001 superfamily, COG5001 super family PBP1_NHase PAS superfamily, and CHD4 Med26_M super family comprise of two BLUF domains at each end with an effector domain sandwiched in-between (Table 1). LOV photoreceptors have also been reported to be present in two copies, along with effector sequences, but both are situated at the N-terminus of effector domains [61]. It remains to be seen if the role of two BLUF domains on either side of the effector is to diversify the signal further or to play the role of a signal transduction further downstream with the help of the auxiliary linker helix found toward the C-terminus of the conserved BLUF domain in our database. The function of these short auxillary helical stretches, located in the association of several BLUF and LOV photoreceptors, is not known precisely, but it seems likely that they mediate the signal progression between their photosensors and the effector domain, as has been predicted in earlier reports [62]. Figure 5: Representation of 3D structural models of BLUF (Red) and effector domain combinations (Blue) using the Phyre server [54]. **(A)** WP_0229622806.1 (_Pseudomonas pelagic_) (206 aa) codes a combination of the BLUF (5–101) and DUF domain (99–181). The model was generated with 100% confidence covering 1–186 residues using the _R. sphaeroides_ AppA (PDB: 4 HH0) as the template. **(B)** WP_051596720.1 (_Curbobacterium sp_. UNCCL17) (470 aa) codes for BLUF in combination with the transcriptional repressor (224–327). The model was generated with \(>\)90% confidence covering 1–350 residues using 12 different templates. **(C)** Q8S9F2.1 (_Euglen gracilis_) (1019 aa) codes for BLUF in combination with cyclase homology domains (CHDs), which are part of the class III nucleotydylcyclases (20–379). The model was generated with \(>\)90% confidence covering 1–800 residues using 13 different templates. **(D)** ARH96915.1 (_Escherichia coli_) (403 aa) codes for BLUF (2–93) in combination with the EAL signaling domain (150–389). The model was generated with 100% confidence covering 1–389 residues using _K. pneumoniae_ BlrP1 (PDB: 3 GFZ) as the template. The structures are represented as interactive coloured ribbons. The model images were generated using PyMol ([[http://www.ppmol.org](http://www.ppmol.org)]([http://www.ppmol.org](http://www.ppmol.org))) [63].