Flavodiiron Proteins in Oxygenic Photosynthetic Organisms: Photoprotection of Photosystem II by Flv2 and Flv4 in Synechocystis sp. PCC 6803

BACKGROUND: Flavodiiron proteins (FDPs) comprise a group of modular enzymes that function in oxygen and nitric oxide detoxification in Bacteria and Archaea. The FDPs in cyanobacteria have an extra domain as compared to major prokaryotic enzymes. The physiological role of cyanobacteria FDPs is mostly unknown. Of the four putative flavodiiron proteins (Flv1-4) in the cyanobacterium Synechocystis sp. PCC 6803, a physiological function in Mehler reaction has been suggested for Flv1 and Flv3. PRINCIPAL FINDINGS: We demonstrate a novel and crucial function for Flv2 and Flv4 in photoprotection of photosystem II (PSII) in Synechocystis. It is shown that the expression of Flv2 and Flv4 is high under air level of CO(2) and negligible at elevated CO(2). Moreover, the rate of accumulation of flv2 and flv4 transcripts upon shift of cells from high to low CO(2) is strongly dependent on light intensity. Characterization of FDP inactivation mutants of Synechocystis revealed a specific decline in PSII centers and impaired translation of the D1 protein in Delta flv2 and Delta flv4 when grown at air level CO(2) whereas at high CO(2) the Flvs were dispensable. Delta flv2 and Delta flv4 were also more susceptible to high light induced inhibition of PSII than WT or Delta flv1 and Delta flv3. SIGNIFICANCE: Analysis of published sequences revealed the presence of cyanobacteria-like FDPs also in some oxygenic photosynthetic eukaryotes like green algae, mosses and lycophytes. Our data provide evidence that Flv2 and Flv4 have an important role in photoprotection of water-splitting PSII against oxidative stress when the cells are acclimated to air level CO(2). It is conceivable that the function of FDPs has changed during evolution from protection against oxygen in anaerobic microbes to protection against reactive oxygen species thus making the sustainable function of oxygen evolving PSII possible. Higher plants lack FDPs and distinctly different mechanisms have evolved for photoprotection of PSII

Single particle analysis of thylakoid proteins from Thermosynechococcus elongatus and Synechocystis 6803: Localization of the CupA subunit of NDH-1

The larger protein complexes of the cyanobacterial photosynthetic membrane of Thermosynechoccus elongatus and Synechocystis 6803 were studied by single particle electron microscopy after detergent solubilization, without any purification steps. Besides the ‘‘standard’’ L-shaped NDH-1L complex, related to complex I, large numbers of a U-shaped NDH-1MS complex were found in both cyanobacteria. In membranes from Synechocystis ΔcupA and ΔcupA/cupB mutants the U-shaped complexes were absent, indicating that CupA is responsible for the U-shape by binding at the tip of the membrane-bound arm of NDH-1MS. Comparison of membranes grown under air levels of CO2 or 3% CO2 indicates that the number of NDH-1MS particles is 30-fold higher under low-CO2.

In situ dynamics of O2, pH and cyanobacterial transcripts associated with CCM, photosynthesis and detoxification of ROS

The relative abundance of transcripts encoding proteins involved in inorganic carbon concentrating mechanisms (CCM), detoxification of reactive oxygen species (ROS) and photosynthesis in the thermophilic cyanobacterium Synechococcus OS-B′ was measured in hot spring microbial mats over two diel cycles, and was coupled with in situ determinations of incoming irradiance and microenvironmental dynamics of O2 and pH. Fluctuations in pH and O2 in the mats were largely driven by the diel cycle of solar irradiance, with a pH variation from ∼7.0 to ∼9.5, and O2 levels ranging from anoxia to supersaturation during night and day, respectively. Levels of various transcripts from mat cyanobacteria revealed several patterns that correlated with incident irradiance, O2 and pH within the mat matrix. Transcript abundances for most genes increased during the morning dark–light transition. Some transcripts remained at a near constant level throughout the light period, whereas others showed an additional increase in abundance as the mat underwent transition from low-to-high light (potentially reflecting changes in O2 concentration and pH), followed by either a decreased abundance in the early afternoon, or a gradual decline during the early afternoon and into the evening. One specific transcipt, psbA1, was the lowest during mid-day under high irradiance and increased when the light levels declined. We discuss these complex in situ transcriptional patterns with respect to environmental and endogenous cues that might impact and regulate transcription over the diel cycle