Environment of TyrZ in photosystem II from Thermosynechococcus elongatus in which PsbA2 is the D1 protein

The main cofactors that determine the photosystem II (PSII) oxygen evolution activity are borne by the D1 and D2 subunits. In the cyanobacterium Thermosynechococcus elongatus, there are three psbA genes coding for D1. Among the 344 residues constituting D1, there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2, and 27 between PsbA2 and PsbA3. Here, we present the first study of PsbA2-PSII. Using EPR and UV-visible time-resolved absorption spectroscopy, we show that: (i) the time-resolved EPR spectrum of Tyr_Z^• in the (S₃Tyr_Z^•)′ is slightly modified; (ii) the split EPR signal arising from Tyr_Z^• in the (S₂Tyr_Z^•)′ state induced by near-infrared illumination at 4.2 K of the S₃Tyr_Z state is significantly modified; and (iii) the slow phases of P₆₈₀^+⋅ reduction by Tyr_Z are slowed down from the hundreds of μs time range to the ms time range, whereas both the S₁Tyr_Z^• → S₂Tyr_Z and the S₃Tyr_Z^• → S₀Tyr_Z + O₂ transition kinetics remained similar to those in PsbA(1/3)-PSII. These results show that the geometry of the Tyr_Z phenol and its environment, likely the Tyr-O···H···Nϵ-His bonding, are modified in PsbA2-PSII when compared with PsbA(1/3)-PSII. They also point to the dynamics of the proton-coupled electron transfer processes associated with the oxidation of Tyr_Z being affected. From sequence comparison, we propose that the C144P and P173M substitutions in PsbA2-PSII versus PsbA(1/3)-PSII, respectively located upstream of the α-helix bearing Tyr_Z and between the two α-helices bearing Tyr_Z and its hydrogen-bonded partner, His-190, are responsible for these changes.     

Changes in composition of membrane proteins accompanying the regulation of PS I/PS II stoichiometry observed with Synechocystis PCC 6803

Changes in composition of membrane proteins in Synechocystis PCC 6803 induced by the shift of light regime for photosynthetic growth were studied in relation to the regulation of PS I/PS II stoichiometry. Special attention was paid to the changes in abundance of proteins of PS I and PS II complexes. Composition was examined using a LDS-PAGE and a quantitative enzyme immunoassay. Abundance of PsaA/B polypeptides and the PsaC polypeptide of the PS I complex, on a per cell basis, increased under the light regime exciting preferentially PS II and decreased under the light regime exciting mainly PS I. Similar changes were observed with polypeptides of 18.5, 10 and 8.5 kDa. The abundance of other proteins associated with membranes, including PsbA polypeptide of the PS II complex, was fairly constant irrespective of light regime. These results are consistent with our previous observations with other strains of cyanophytes (Anabaena variabilis M2 and Synechocystis PCC 6714) that PS I is the variable component in changes in PS I/PS II stoichiometry in response to changing light regimes for photosynthesis.Abbreviations CBB Coomassie brilliant blue - Chl chlorophyll - EIA enzyme immunoassay - LDS lithium dodecyl sulfate - PAGE polyacrylamide gel electrophoresis - PS photosystem - PVDF polyvinylidene difluoride     

Effects of Temperature and Nutrient Supply on Resource Allocation,Photosynthetic Strategy,and Metabolic Rates of Synechococcus sp.

Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but their role is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (Synechococcus PCC 7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO, rbcL) and light (Photosystem II, psbA) photosynthetic reactions, the content of chlorophyll a, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found that rbcL and psbA abundance increased with nutrient supply, whereas a temperature-induced increase in psbA occurred only in nutrient-replete treatments. Low temperature and abundant nutrients caused increased RuBisCO abundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient-sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates in Synechococcus sp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark-reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments.     

The mechanism of photosystem‐II inactivation during sulphur deprivation‐induced H2 production in Chlamydomonas reinhardtii

Sulphur limitation may restrain cell growth and viability. In the green alga Chlamydomonas reinhardtii, sulphur limitation may induce H₂ production lasting for several days, which can be exploited as a renewable energy source. Sulphur limitation causes a large number of physiological changes, including the inactivation of photosystem II (PSII), leading to the establishment of hypoxia, essential for the increase in hydrogenase expression and activity. The inactivation of PSII has long been assumed to be caused by the sulphur‐limited turnover of its reaction center protein PsbA. Here we reinvestigated this issue in detail and show that: (i) upon transferring Chlamydomonas cells to sulphur‐free media, the cellular sulphur content decreases only by about 25%; (ii) as demonstrated by lincomycin treatments, PsbA has a significant turnover, and other photosynthetic subunits, namely RbcL and CP43, are degraded more rapidly than PsbA. On the other hand, sulphur limitation imposes oxidative stress early on, most probably involving the formation of singlet oxygen in PSII, which leads to an increase in the expression of GDP‐L‐galactose phosphorylase, playing an essential role in ascorbate biosynthesis. When accumulated to the millimolar concentration range, ascorbate may inactivate the oxygen‐evolving complex and provide electrons to PSII, albeit at a low rate. In the absence of a functional donor side and sufficient electron transport, PSII reaction centers are inactivated and degraded. We therefore demonstrate that the inactivation of PSII is a complex and multistep process, which may serve to mitigate the damaging effects of sulphur limitation.     

Genetic diversity among Synechococcus spp. (cyanobacteria) isolated from the pelagial of Lake Constance

Abstract: Seven phycoerythrin (PE)-rich and six phycocyanin (PC)-rich unicellular cyanobacteria of the Synechococcus type were isolated from the pelagial of Lake Constance. The genetic diversity among the isolates was evaluated using restriction fragment length polymorphism (RFLP) within the psbA gene family. Nine out of 13 isolates exhibit different DNA structures in the probed areas and, furthermore, they differ from morphologically similar strains collected from other lakes. The data set does not support a principal distinction between PC-rich and PE-rich strains but it reveals less heterogeneity in the coding region of the psbA genes among PE-rich isolates than among PC-rich isolates. The isolation of distinct strains in different seasons suggests species diversity and seasonal occurrence of Synechococcus spp. in Lake Constance.