Isolation of a photosystem 2 preparation from higher plants with highly enriched oxygen evolution activity

AbstractDetergent-treatment of higher plant thylakoids with Triton X-100 at pH 6.3 has been used to purify a PS2 fraction with very high rates of oxygen evolution (1000 μmol.mg chl−1.h−1). A photosynthetic unit size of about 300 chlorophyll (chl) molecules has been determined by optical methods, suggesting an average turnover time for PS2 of about 2 ms. The donor system for P680+ is particularly well preserved in the preparation, as judged by P680+ reduction kinetics, the detection by EPR of Signal IILT and the presence of the high potential form of cytochrome b-559 (at a ratio of 1:1 with the reaction centre)

Rapid electron transfer reactions associated with oxygen evolution in photosystem II preparations from spinach and Phormidium laminosum

AbstractWe have measured the nanosecond kinetics of Chl-a+II reduction in oxygen-evolving detergent preparations of PS II from the cyanobacterium Phormidium laminosum and from higher plants (spinach) at 824 and 680 nm. Compared to earlier studies at 680 nm with higher plant material, we obtained an improved signal: noise ratio for measurements on a ns to ms time scale. The kinetics of Chl-a+II reduction in the ns range are consistent in the two preparations and are comparable to other studies of higher plant and cyanobacterial material. The ns kinetics are tightly connected to the ability for O2 evolution. Analysis of the μs kinetics indicates three phases: (a) the slow phase (t12 ~ 150 μs in spinach and ~ 500 μs in Phormidium) reflects the back reaction between Chl-a+II and Q−; (b) the phase with t125–10 μs is probably due to a donor which is not connected to an intact water oxidation system; (c) the intermediate μs component (t12 30–40 μs) may be related to water oxidation

Electron donation by the high-potential haems in Rhodopseudomonas viridis reaction centres at low temperatures

AbstractEPR signals due to c-type cytochromes in Rhodopseudomonas viridis reaction centres are reported. Haems with Em = 380 mV (1), 310 mV (1) and about 0 mV (2) were identified. At redox potentials where the low-potential cytochromes are oxidised, but the high-potential cytochromes are reduced, photooxidation of the high-potential cytochromes is observed as QA is photoreduced by low-temperature (15 K) illumination. Cyt c-556→cyt c-559→reaction centre→QA However, the signal attributed to QA is only 30–40% of the intensity and is narrower than that observed when QA photoreduced with all the haems reduced. It is suggested that reduction of the low-potential haems causes conformational changes in the reaction centre, altering the iron-quinone interaction and the electron-transfer process in the cytochrome chain

A reminder before extinction strengthens episodic memory via reconsolidation but fails to disrupt generalized threat responses

Contains fulltext : 177015.pdf (publisher's version ) (Open Access)A reminder can temporarily renew flexibility of consolidated memories, referred to as reconsolidation. Pavlovian threat-conditioning studies suggest that a reminder can renew flexibility of threat responses but that episodic memories remain stable. In contrast, outside the threat-conditioning domain, studies testing memory for word lists or stories find that a reminder can renew flexibility of episodic memory. This discrepancy in findings leaves it unclear if episodic memories reconsolidate, or only Pavlovian responses. Here we unite the different approaches in the field and show that a reminder can retroactively strengthen episodic memory for Pavlovian threat-conditioned events, but that, in contrast to threat-conditioning studies with simple sensory stimuli, extinction after a reminder fails to prevent recovery of generalized threat responses. Our results indicate the episodic memories also reconsolidate, allowing strengthening of relevant memories. These findings also suggest that generalized threat responses and episodic memories are less susceptible to be modified by reminder-interventions procedures

IDENTIFICATION OF MEMBRANE BOUND FERREDOXIN-LIKE CENTRES IN PHOTOSYSTEM I OF BLUE-GREEN ALGAE BY MÖSSBAUER AND EPR SPECTROSCOPY

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