Site, Rate, and Mechanism of Photoprotective Quenching in Cyanobacteria

Abstract

In cyanobacteria, activation of the Orange Carotenoid Protein (OCP) by intense blue-green light triggers photoprotective thermal dissipation of excess absorbed energy leading to a decrease (quenching) of fluorescence of the light harvesting phycobilisomes and, concomitantly, of the energy arriving to the reaction centers. Using spectrally resolved picosecond fluorescence, we have studied cells of wild-type Synechocystis sp. PCC 6803 and of mutants without and with extra OCP (ΔOCP and OverOCP) both in the unquenched and quenched state. With the use of target analysis, we managed to spectrally resolve seven different pigment pools in the phycobilisomes and photosystems I and II, and to determine the rates of excitation energy transfer between them. In addition, the fraction of quenched phycobilisomes and the rates of charge separation and quenching were resolved. Under our illumination conditions, ∼72% of the phycobilisomes in OverOCP appeared to be substantially quenched. For wild-type cells, this number was only ∼29%. It is revealed that upon OCP activation, a bilin chromophore in the core of the phycobilisome, here called APC(Q)(660), with fluorescence maximum at 660 nm becomes an effective quencher that prevents more than 80% of the excitations in the phycobilisome to reach Photosystems I and II. The quenching rate of its excited state is extremely fast, that is, at least (∼240 ± 60 fs)(-1). It is concluded that the quenching is most likely caused by charge transfer between APC(Q)(660) and the OCP carotenoid hECN in its activated form.

The Cyanobacterial Photoactive Orange Carotenoid Protein Is an Excellent Singlet Oxygen Quencher

“…Our results clearly demonstrate that OCP protects the photosystems and cells via two different mechanisms: by quenching of excess excitation energy (Wilson et al, 2006;Rakhimberdieva et al, 2010;Tian et al, 2011), and thereby decreasing charge separations at the reaction centers, and by quenching 1 O 2 , which is formed during the light reactions that still occur in the reaction centers and in the antennae. These two protecting mechanisms complement each other.…”

Flavodiiron Protein Flv2/Flv4-Related Photoprotective Mechanism Dissipates Excitation Pressure of PSII in Cooperation with Phycobilisomes in Cyanobacteria

“…OCP-mediated NPQ is more active in HL conditions (El Bissati et al, 2000), whereas we have shown that the Flv2/Flv4-mediated electron transfer mechanism is active in LC conditions already at GL. Moreover, upon exposition of cells to 350 mmol photons m 22 s 21 of blue light for 1 h, only about 29% of the PBs were quenched by the OCP mechanism in vivo (Tian et al, 2011). This observation implies that the Flv2/Flv4 heterodimer might be important at the acceptor side of those PSII complexes that received a correct energy transfer from PBs.…”

Structural and Functional Modularity of the Orange Carotenoid Protein: Distinct Roles for the N- and C-Terminal Domains in Cyanobacterial Photoprotection

“…The C-terminal domain cannot function as a photosensor without a light absorbing chromophore. The N-terminal effector domain, when bound to the PB, requires 39-hECN to directly dissipate energy in the OCP induced photoprotective mechanism (Tian et al, 2011(Tian et al, , 2012Stadnichuk et al, 2012). The OCP thus represents a remarkably minimized system in which sensor and effector domains share a single chromophore that is critical to both photosensory and photoprotective function.…”