Abstract
Cyanobacteria harbor GAF-containing photoreceptors that may bind a linear tetrapyrrole as a chromophore (cyanobacteriochrome). We have reported that the GAF domain of the cyanobacteriochrome TePixJ of Thermosynechococcus elongatus binds phycoviolobilin covalently and shows reversible photoconversion between blue and green-absorbing forms. We also demonstrated that the TePixJ_GAF apoprotein can be reconstituted with phycocyanobilin to form an intermediary photoactive complex. Here, we studied the role of the apoprotein for photoconversion of TePixJ_GAF. We found a conserved cystein residue among cyanobacteriochromes that show photoconversion between blue and green-absorbing forms. We mutagenized this residue in TePixJ_GAF and found that the mutant protein is assembled with phycocyanobilin to form a red-absorbing photoactive complex. FTIR spectroscopy of wild type TePixJ_GAF revealed green light-induced crosslinking of a free SH group. These results suggest that the light-induced crosslinking of the cystein residue to the chromophore is essential for assembly of the blue-absorbing form.
