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In Synechocystis sp. PCC 6803, the flv4-2 operon encodes the flavodiiron proteins Flv2 and Flv4 together with a small protein, Sll0218, providing photoprotection for Photosystem II (PSII). Here, the distinct roles of Flv2/Flv4 and Sll0218 were addressed, using a number of flv4-2 operon mutants. In the sll0218 mutant, the presence of Flv2/Flv4 rescued PSII functionality as compared with sll0218-flv2, where neither Sll0218 nor the Flv2/Flv4 heterodimer are expressed. Nevertheless, both the sll0218 and sll0218-flv2 mutants demonstrated deficiency in accumulation of PSII proteins suggesting a role for Sll0218 in PSII stabilization, which was further supported by photoinhibition experiments. Moreover, the accumulation of PSII assembly intermediates occurred in Sll0218-lacking mutants. The YFP-tagged Sll0218 protein localized in a few spots per cell at the external side of the thylakoid membrane, and biochemical membrane fractionation revealed clear enrichment of Sll0218 in the PratA-defined membranes, where the early biogenesis steps of PSII occur. Further, the characteristic antenna uncoupling feature of the flv4-2 operon mutants is shown to be related to PSII destabilization in the absence of Sll0218. It is concluded that the Flv2/Flv4 heterodimer supports PSII functionality, while the Sll0218 protein assists PSII assembly and stabilization, including optimization of light harvesting. This work clarifies and dissects the roles of the flv4-2 operon-encoded proteins, Flv2/Flv4 heterodimer and the elusive Sll0218, in photoprotection of the photosynthetic apparatus in Synechosystis. While Flv2/Flv4 heterodimer is involved in an alternative electron transfer route, the Sll0218 protein is localized to specific cell compartments where photosynthetic complexes are assembled, and it is involved in the stabilization of Photosystem II complexes.

With the tremendous progress of the past decades, molecular plant science is becoming more unified than ever. We now have the exciting opportunity to further connect subdisciplines and understand plants as whole organisms, as will be required to efficiently utilize them in natural and agricultural systems to meet human needs. The subfields of photosynthesis, plant developmental biology and plant stress are used as examples to discuss how plant science can become better integrated. The challenges, strategies and rich opportunities for the integration of the plant sciences are discussed. In recent years, more and more overlap between various subdisciplines has been inadvertently discovered including tradeoffs that may occur in plants engineered for biotechnological applications. Already important, bioinformatics and computational modelling will become even more central to structuring and understanding the ever growing amounts of data. The process of integrating and overlapping fields in plant biology research is advancing, but plant science will benefit from dedicating more effort and urgency to reach across its boundaries.

Screening of 55 different cyanobacterial strains revealed that an extract from Nostoc XPORK14A drastically modifies the amplitude and kinetics of chlorophyll a fluorescence induction of Synechocystis PCC6803 cells.After 2 d exposure to the Nostoc XPORK14A extract, Synechocystis PCC 6803 cells displayed reduced net photosynthetic activity and significantly modified electron transport properties of photosystem II under both light and dark conditions. However, the maximum oxidizable amount of P700 was not strongly affected. The extract also induced strong oxidative stress in Synechocystis PCC 6803 cells in both light and darkness. We identified the secondary metabolite of Nostoc XPORK14A causing these pronounced effects on Synechocystis cells. Mass spectrometry and nuclear magnetic resonance analyses revealed that this compound, designated as M22, has a non-peptide structure. We propose that M22 possesses a dualaction mechanism: firstly, by photogeneration of reactive oxygen species in the presence of light, which in turn affects the photosynthetic machinery of Synechocystis PCC 6803; and secondly, by altering the in vivo redox status of cells, possibly through inhibition of protein kinases.

In cyanobacteria, the NAD(P)H:quinone oxidoreductase (NDH-1) is involved in a variety of functions like respiration, cyclic electron flow around PSI and CO(2) uptake. Several types of NDH-1 complexes, which differ in structure and are responsible for these functions, exist in cyanobacterial membranes. This minireview is based on data obtained by reverse genetics and proteomics studies and focuses on the structural and functional differences of the two types of cyanobacterial NDH-1 complexes: NDH-1L, important for respiration and PSI cyclic electron flow, and NDH-1MS, the low-CO(2) inducible complex participating in CO(2) uptake. The NDH-1 complexes in cyanobacteria share a common NDH-1M 'core' complex and differ in the composition of the distal membrane domain composed of specific NdhD and NdhF proteins, which in complexes involved in CO(2) uptake is further associated with the hydrophilic carbon uptake (CUP) domain. At present, however, very important questions concerning the nature of catalytically active subunits that constitute the electron input device (like NADH dehydrogenase module of the eubacterial 'model' NDH-1 analogs), the substrate specificity and reaction mechanisms of cyanobacterial complexes remain unanswered and are shortly discussed here.

Cyanobacteria possess a complex CO(2)-concentrating mechanism (CCM), which is induced by low inorganic carbon conditions. To investigate the involvement of proteases in the processes of induction and degradation of the CCM complexes, we studied the FtsH2 (DeltaSlr0228) and Deg-G (DeltaSlr1204/DeltaSll1679/DeltaSll1427) protease mutants of Synechocystis sp. PCC 6803. WT and protease mutant cells were grown under high CO(2) and then shifted to low CO(2), followed by a proteome analysis of the membrane protein complexes. Interestingly, in the FtsH2 protease mutant, inducible CCM complexes were not detected upon shift to low CO(2), whereas the Deg-G mutant behaved like WT. Also the transcripts of the inducible CCM genes and their regulator ndhR failed to accumulate upon shift of FtsH2 mutant cells from high to low CO(2), indicating that the regulation by the FtsH2 protease is upstream of NdhR. Moreover, functional photosynthesis was shown a prerequisite for induction of CCM in WT at low CO(2), possibly via generation of oxidative stress, which was shown here to enhance the expression of inducible CCM genes even at high CO(2) conditions. Once synthesized, the CCM complexes were not subject to proteolytic degradation, even when dispensable upon a shift of cells to high CO(2).

In Arabidopsis thaliana, the chloroplast-targeted enzyme ferredoxin-NADP+-oxidoreductase (FNR) exists as two isoforms, AtLFNR1 and AtLFNR2, encoded by the genes At5g66190 and At1g20020, respectively. Both isoforms are evenly distributed between the thylakoids and soluble stroma, and they are separated by two-dimensional electrophoresis in four distinct spots, suggesting post-translational modification of both isoforms. To reveal the functional specificity of AtLFNR1, we have characterized the T-DNA insertion mutants with an interrupted At5g66190 gene. Absence of AtLFNR1 resulted in a reduced size of the rosette with pale green leaves, which was accompanied by a low content of chlorophyll and light-harvesting complex proteins. Also the photosystem I/photosystem II (PSI/PSII) ratio was significantly lower in the mutant, but the PSII activity, measured as the F(V)/F(M) ratio, remained nearly unchanged and the excitation pressure of PSII was lower in the mutants than in the wild type. A slow re-reduction rate of P700 measured in the mutant plants suggested that AtLFNR1 is involved in PSI-dependent cyclic electron flow. Impaired function of FNR also resulted in decreased capacity for carbon fixation, whereas nitrogen metabolism was upregulated. In the absence of AtLFNR1, we found AtLFNR2 exclusively in the stroma, suggesting that AtLFNR1 is required for membrane attachment of FNR. Structural modeling supports the formation of a AtLFNR1-AtLFNR2 heterodimer that would mediate the membrane attachment of AtLFNR2. Dimer formation, in turn, might regulate the distribution of electrons between the cyclic and linear electron transfer pathways according to environmental cues.

The protein assembly and stability of photosystem II (PSII) (sub)complexes were studied in mature leaves of four plastid mutants of tobacco (Nicotiana tabacum L), each having one of the psbEFLJ operon genes inactivated. In the absence of psbL, no PSII core dimers or PSII–light harvesting complex (LHCII) supercomplexes were formed, and the assembly of CP43 into PSII core monomers was extremely labile. The assembly of CP43 into PSII core monomers was found to be necessary for the assembly of PsbO on the lumenal side of PSII. The two other oxygen-evolving complex (OEC) proteins, PsbP and PsbQ, were completely lacking in ΔpsbL. In the absence of psbJ, both intact PSII core monomers and PSII core dimers harboring the PsbO protein were formed, whereas the LHCII antenna remained detached from the PSII dimers, as demonstrated by 77 K fluorescence measurements and by the lack of PSII–LHCII supercomplexes. The ΔpsbJ mutant was characterized by a deficiency of PsbQ and a complete lack of PsbP. Thus, both the PsbL and PsbJ subunits of PSII are essential for proper assembly of the OEC. The absence of psbE and psbF resulted in a complete absence of all central PSII core and OEC proteins. In contrast, very young, vigorously expanding leaves of all psbEFLJ operon mutants accumulated at least traces of D2, CP43 and the OEC proteins PsbO and PsbQ, implying developmental control of the expression of the PSII core and OEC proteins. Despite severe problems in PSII assembly, the thylakoid membrane complexes other than PSII were present and correctly assembled in all psbEFLJ operon mutants.

Recombinant phosphoserine aminotransferase (EC 2.6.1.52) from Bacillus circulans subsp. alkalophilus was crystallized at room temperature from 0.1 M sodium acetate buffer, pH 4.6, and 2% PEG 20000, using macroseeding techniques. The crystals diffract X-rays to at least 2.0 A nominal resolution. They belong to space group C2 with unit cell dimensions a = 93.2 A, b = 93.1 A, c = 45.6 A, alpha = 90.0 degrees, beta = 106.8 degrees, gamma = 90.0 degrees. A native data set to 2.3 A has been collected. Assuming an average packing density of the crystals, there is one monomer in the asymmetric unit, resulting in a calculated solvent content of 48.2%.