Orange Carotenoid Protein (OCP): A Key Player in Non-photochemical Quenching of Cyanobacteria under Fluctuating Light Condition

Abstract

Fluctuating light condition poses major threat to photosynthetic organisms by evoking the production of reactive oxygen species (ROS). To endure the high irradiance level, plants and algae have evolved a photo-protective mechanism, referred as non-photochemical quenching (NPQ). This mechanism concerns with minimizing arrival of the excess excitation energy on reaction centers by dissipating surplus energy in form of harmless heat. Earlier cyanobacteria were not considered to capable of performing NPQ. Alternatively, state transition was supposed to be the major means that cyanobacteria preferably carried out to be protected under high light. Recently it was substantiated with evidence that these organisms can execute NPQ as a prominent photo-protective strategy. NPQ in cyanobacteria is mediated by a water soluble orange carotenoid protein (OCP) which is structurally and functionally modular. OCP consists of two domains (i) N-terminal domain (NTD) and (ii) C-terminal domain (CTD) with a single carotenoid as a chromophore spanning symmetrically in both domains. Blue-green or strong white light induces conversion of OCP from an inactive orange state (OCPO) to active red state (OCPR). Active form of OCP (OCPR) binds to core of light harvesting antenna complex, phycobilisome (PBS), where it quenches fluorescence and assists in dissipation of excess energy by non-radiative pathway. Prior to prevent wasteful quenching of fluorescence under light starvation, another protein named fluorescence recovery protein (FRP) partakes in decoupling OCPR from PBS and accelerates conversion of OCPR state back to OCPO state.