The cyanobacterium Synechococcus modulates Photosystem II function in response to excitation stress through D1 exchange

Abstract

In this minireview we discuss effects of excitation stress on the molecular organization and function of PS II as induced by high light or low temperature in the cyanobacterium Synechococcus sp. PCC 7942. Synechococcus displays PS II plasticity by transiently replacing the constitutive D1 form (D1:1) with another form (D1:2) upon exposure to excitation stress. The cells thereby counteract photoinhibition by increasing D1 turn over and modulating PS II function. A comparison between the cyanobacterium Synechococcus and plants shows that in cyanobacteria, with their large phycobilisomes, resistance to photoinhibition is mainly through the dynamic properties (D1 turnover and quenching) of the reaction centre. In contrast, plants use antenna quenching in the light-harvesting complex as an important means to protect the reaction center from excessive excitation.

A Transient Exchange of the Photosystem II Reaction Center Protein D1:1 with D1:2 during Low Temperature Stress ofSynechococcus sp. PCC 7942 in the Light Lowers the Redox Potential of QB

“…3C). It has been previously hypothesized that the potential for non-photochemical quenching of PSII reaction centers might protect against excess light (10). More recently the reduction of Q A has been suggested as a major requirement for efficient reaction center quenching (43).…”

“…Because cyanobacteria lack zeaxanthin cycle-dependent antenna quenching (45), the proposed mechanism of energy dissipation would be of primary importance in protecting cyanobacterial cells from photoinhibitory damage when PSII reaction centers are exposed to high excitation pressure induced by exposure to chilling temperatures. The transient exchange of the D1:1 protein, which appears to be the initial response, may thus provide the time required for full cellular acclimation to an increased excitation pressure through modifications in protein (2,6,10) and lipid (24,46) composition.…”

“…D1 form 1 (D1:1) is the preferred form in a cell acclimated to its normal growth environment, but when stressed under either high light or low temperatures D1:1 is exchanged for another form of the D1 protein called D1 form 2 (D1:2) (2, 6). However, this exchange is only transient, and when cells have acclimated to the new growth conditions they revert to D1:1 (10). These shifts are governed by changes in the relative expression of the psbAI gene encoding for D1:1 and the psbAII/III genes encoding for D1:2 (2, 4).…”

“…Both D1:1 and D1:2 have a total of 360 amino acids. Gene-inactivation mutant strains expressing either only D1:1 (R2S2C3) or only D1:2 (R2K1) (11) have proven to be very useful in studies of differential psbA expression in response to environmental changes (10,12). Cells with D1:2 appear to be more stress-resistant than those possessing D1:1 under conditions when the excitation pressure on PSII increases due to either increased irradiance or decreased temperature (2,3,6).…”

Salt stress inhibits photosystems II and I in cyanobacteria

“…Since the efficiency and productivity of photosynthesis are closely related to the activities of the two photosystems, considerable attention has been paid to the effects of environmental stress on these photosystems. Cyanobacteria provide suitable model systems for studies of the effects of environmental stress on photosynthesis since these prokaryotes perform oxygenic photosynthesis using a photosynthetic apparatus similar to that found in chloroplasts of higher plants and algae (Pfenning 1978;Ö quist et al 1995). Moreover, cyanobacterial cells can easily be exposed directly to defined stress conditions in culture (Joset et al 1996;Hagemann and Erdmann 1997) and they are able to acclimate to a wide range of environmental stresses (Nishida and Murata 1996;Hagemann and Erdmann 1997).…”