**4.2 Dual cays residues in CBCR for dual photocycle**

Some CBCRs with exceptionally unchanged DXCF motif or the feebly rationed CXXR/K motif have extra Cys amino acids in the insertion loop (embed - Cys) via second thioether bond at the C10 atom under dark phase [52]. This sort of double Cys CBCRs, with a second thioether bond, is fragile and light-labile. These CBCRs are extremely responsive to capture violet or blue light in dark phase but it absorbs green, yellow, orange or greenish-blue light in the light phase. The cyanobacterial CBCRs are primarily linked to PCB yet some may link to phycoviolobilin (PVB) like Cphs [53, 54]. The change of PCB into PVB is unique to the DXCF-CBCRs subfamily [22]. The color tuning systems of CBCRs for far-red to orange (Fr/O) remain unidentified [55].

## **4.3 Signal transmission by CBCR**

Cyanobacterial photoreceptors associated with signal transmission through phosphor transfer or c-di-GMP. Phosphorelay is a signal transmission process engaged with the autophosphorylation of His amino acid residue by His kinases, continued by phosphotransfer in association with reaction controllers. A film bound His kinase CBCR-UirS in *Synechocystis* accompanied with the reaction controller AraC family and UirR roles as a UV absorbing two-segment signaling framework [38]. Signaling in the chromophorylation process is regulated by the cystathionine beta-synthase (CBS) in the N-terminal of SesA. This in SesA can bind to ATP, ADP, and AMP which regulate the signaling process in chromophorylation.

## **4.4 Autolyase and autoisomerase in CBCR**

Cyanobacterial photoreceptors are also called CBCRs that are similar to phytochromes [56]. PixJ GAF, from a thermophilic cyanobacterium *Thermosynechococcus elongatus*, regulates phototaxis. The BP-1 bacterial photoreceptors (TePixJ\_GAF) reveal reversible photoconversion between a blue light (433 nm) capturer and a green light (531 nm) capturer. TePixJ GAF chromoprotein expressed in *Synechocystis* was denatured using acidic urea (8 M urea/HCl, pH 2.0) and it was compared with the cyanobacterial phytochrome Cphl having chromophore phycocyanobilin (PCB). The PCB is not a chromophore part in TePixJ, but PCB is a part of its isomer, phycoviolobilin (PVB). It confers the autolyase and autoisomerase property of GAF in TePixJ.

The primary CBCR for the phototaxis controller was recognized as PixJ. The CBCR SyPixJl of *Synechocystis* sp. PCC 6803 and TePixJ of *Thermosynechococcus elongatus* BP-1 showed selective reverse photo transfiguration between blue absorber (425-435 nm) Pb to green (531–535 nm) absorber Pg [57, 58]. Genetic modification in the pixJ of SypixJl and SypixD lost progressive phototaxis, these CBCR in original structure perceive blue light and characterize the order of motility as a regulatory switch [59]. The anticipated secondary arrangement of SyPixJl has N-terminal transmembrane helices, two successive GAF domains and a C-terminal methyl-accepting structure [5]. Proteolytic destruction and mass spectrometric investigation of SyPixJ 1\_GAF and TePixJ\_GAF showed that a straight tetrapyrrole was covalently bound to a peptide connected with phytochrome, a moderated Cys-His motif [5].

At the point when His6-TePixJ\_GAF was digested with acidic urea in the dark phase, the Pb peak (433 nm) was changed from native form to a peak at 594 nm with a shoulder at 565 nm. The PVB in TePixJ\_GAF captures a shorter wavelength of light

than PCB. In any case, it ought to be noticed that the Pb absorb at 433 nm in native form is extraordinarily smaller than the urea-denatured PVB absorb at 594 nm. PVB is an isomer of PCB with a similar atomic mass, yet conjugated double bonds are detached at the C5 position. PVB with the apoprotein is accountable for the extraordinary blue capturing structure Pb and photoreversible modifications. The PCB transformation to PVB is because of the PecE and PecF proteins which are fundamental for ligation and isomerization.
