Abstract
Cyanobacteria harbor many GAF-containing photoreceptors that bind a linear tetrapyrrole as a chromophore (cyanobacteriochrome). We have reported that the cyanobacteriochrome TePixJ of Thermosynechococcus elongatus: (1) TePixJ exhibits reversible photoconversion between blue and green-absorbing forms, (2) TePixJ covalently binds phycoviolobilin (PVB), which is isomerized from phycocyanobilin (PCB) by TePixJ itself. Here, we performed site-directed mutagenesis of some conserved amino acid residues and expressed the mutant proteins in Escherichia coli producing PCB. E497A mutant showed photoconversion and isomerization, which are comparable to the wild type but a small amount of novel photoinactive species were generated during photoconversion. D492A mutant did not isomerize PCB to PVB and PCB-bound protein showed photoconversion much less efficiently than the wild-type protein. H523A mutant has mostly PVB but showed unusual photoconversion from blue to orange/red-absorbing form. These suggest that hydrogen bonds interacting with the chromophore are critical for the photoconversion of TePixJ and the isomerization from PCB to PVB.
