The cyanobacterium Synechococcus resists UV-B by exchanging photosystem II reaction-center D1 proteins

Abstract

Current ambient UV-B levels can significantly depress productivity in aquatic habitats, largely because UV-B inhibits several steps of photosynthesis, including the photooxidation of water catalyzed by photosystem II. We show that upon UV-B exposure the cyanobacterium Synechococcus sp. PCC 7942 rapidly changes the expression of a family of three psbA genes encoding photosystem II D1 proteins. In wild-type cells the psbAI gene is expressed constitutively, but strong accumulations of psbAII and psbAIII transcripts are In oxygenic photobionts, photosystem II (PSII) is an integral membrane complex that catalyzes the photooxidation of water, with concomitant release of oxygen. Electrons extracted from water are passed to plastoquinone and enter the photosynthetic electron transport chain. The core of the PSII complex is composed of a dimer of two related proteins, D1 and D2, that bind the pigments and cofactors involved in this electron transfer from water to plastoquinone. During active photosynthesis the D1 protein, and to a lesser extent D2, turn over rapidly and are replaced by newly synthesized polypeptides in a PSII repair cycle. Under environmental stress, the repair cycle can be impaired, such that degradation and loss of D1 protein exceeds the rate of replacement (1). This net loss of functional D1 leads to a drop in PSII function and can contribute to photoinhibition, a light-dependent drop in the quantum yield of photosynthesis. Photoinhibition usually occurs when excitation capture exceeds the rate of electron removal from the PSII complex, as can occur when the light intensity exceeds the acclimated irradiance or when the temperature drops below the acclimated level.

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

“…The appearance of the D1:2 copy might have contributed to the elimination of the flv4-2 operon relatively early during evolution (Table II). The D1 protein from algae and plants, which is encoded by a single psbA gene, shares more common residues with the D1:2 cyanobacterial D1 copy, including the 130E residue (Svensson et al, 1991;Winhauer et al, 1991;Campbell et al, 1998b;Takishita and Uchida, 1999). These observations suggest that the Q130E D1 form was a successful strategy for the photosynthetic organisms, becoming ubiquitous among them.…”

Differential response of planktonic primary, bacterial, and dimethylsulfide production rates to static vs. dynamic light exposure in upper mixed-layer summer sea waters

“…However, this is not clearly supported by the radiative stress indicators measured. In addition, repair is thought to be more efficient at elevated temperatures like those encountered in our study (Campbell et al, 1998;van de Poll and Buma, 2009). The fact that surface inhibition was only moderate and that highest PPp occurred in the middle bottle suggests that the photosynthetic machinery of phytoplankton was well adapted to a stratified system and thus not geared to take advantage of fast changes in spectral irradiance.…”

Ultraviolet radiation shapes seaweed communities

“…and Synechococcus sp. In both species, it was found that exposure to moderate doses of UVB results in an increased turnover rate of the D 1 and D 2 reaction centre subunits of PS II, thus, rapidly replacing damaged protein by newly synthesised polypeptides (Campbell et al 1998;Má té et al 1998). The latter authors found that UVB induces the transcription of psbA genes, which encode the D 1 reaction centre protein of PS II.…”