Abstract
Cyanobacteria harbor GAF-containing photoreceptors that may bind a linear tetrapyrrole as a chromophore (cyanobacteriochrome). Previously, we reported the properties of the GAF domain of TePixJ of Thermosynechococcus elongatus that was expressed in Synechocystis. Purified holoprotein showed reversible photoconversion between the 433nm and 531nm-absorbing forms. Denaturation analysis showed that the chromophore of TePixJ is phycoviolobilin. Here, we report reconstitution of TePixJ_GAF apoprotein with synthetic phycocyanobilin. In the beginning of reconstitution, free phycocyanobilin was covalently bound to the apoprotein with concomitant formation of photoactive holoprotein between 430 and 545 nm-absorbing forms. Extensive incubation resulted in conversion to phycoviolobilin which shows photoconversion between 434 and 532 nm in holoprotein. These suggest that phycocyanobilin initially incorporated into TePixJ_GAF is distorted, leading to auto-isomerization to phycoviolobilin. Thus, we can conclude that the GAF domain of TePixJ has a triple function: lyase, isomerase and photoconversion.
