Protonation state of chromophore determines the light absorbing form of chromatic adaptation sensors.

Abstract

In certain cyanobacteria, green-light irradiation induces accumulation of a green-absorbing pigment, phycoerythrin, whereas red-light irradiation induces accumulation of a red-absorbing pigment, phycocyanin, in the photosynthetic light-harvesting phycobilisome. Recent studies have revealed that certain kind of phytochrome-like photoreceptor (cyanobacteriochrome), which photoconverts between green-absorbing form (Pg) and red-light absorbing form (Pr), regulates this phenomenon. Here, we demonstrated that the conversion between Pg and Pr is coupled with protonation and deprotonation of phycocyanobilin (PCB) chromophore. Namely, we proposed that PCB is deprotonated in Pg, whereas it is protonated in Pr. This mechanism is apparently distinct from that of phytochrome, where the chromophore is protonated in both Pr and far-red absorbing form (Pfr).