State 1 and State 2 in Photosynthetic Apparatus of Red Microalgae and Cyanobacteria.

Imbalanced light absorption by photosystem I (PSI) and photosystem II (PSII) in oxygenic phototrophs leads to changes in interaction of photosystems altering the linear electron flow. In plants and green algae, this imbalance is mitigated by a partial migration of the chlorophyll a/b containing light-harvesting antenna between the two photosystem core complexes. This migration is registered as fluorescence changes of the pigment apparatus and is termed the reverse transitions between States 1 and 2. By contrast, the molecular mechanism of State 1/2 transitions in phycobilisome (PBS)-containing photosynthetics, cyanobacteria and red algae, is still insufficiently understood. The suggested hypotheses - PBS movement along the surface of thylakoid membrane between PSI and PSII complexes, reversible PBS detachment from the dimeric PSII complex, and spillover - have some limitations as they do not fully explain the accumulated data. Here, we have recorded changes in the stationary fluorescence emission spectra of red algae and cyanobacteria in States 1/2 at room temperature, which allowed us to offer an explanation of the existing contradictions. The change of room temperature fluorescence of chlorophyll belonged to PSII was revealed, while the fluorescence of PBS associated with the PSII complexes remained during States 1/2 transitions at the stable level. Only the reversible dissociation of PBS from the monomeric PSI was revealed earlier which implied different degree of surface contact of PBS with the two photosystems. The detachment of PBS from the PSI corresponds to ferredoxin oxidation as electron carrier and the increase of cyclic electron transport in the pigment apparatus in State I.

Role of the PB-loop in ApcE and phycobilisome core function in cyanobacterium Synechocystis sp. PCC 6803.

The phycobilisome (PBS) is a giant highly-structured pigment-protein antenna of cyanobacteria and red algae. PBS is composed of the phycobiliproteins and several linker polypeptides. The large core-membrane linker protein (LCM or ApcE) influences many features and functions of PBS and consists of several domains including the chromophorylated PB-domain. Being homologous to the phycobiliprotein α-subunits this domain includes a so-called PB-loop insertion whose functions are still unknown. We have created the photoautotrophic mutant strain of the cyanobacterium Synechocystis sp. PCC 6803 with lacking PB-loop. Using various spectral techniques we have demonstrated that this mutation does not destroy the PBS integrity and the internal PBS excitation energy transfer pathways. At the same time, the deletion of the PB-loop leads to the decrease of connectivity between the PBS and thylakoid membrane and to the compensatory increase of the relative photosystem II content. Mutation provokes the violation of the thylakoid membranes arrangement, the inability to perform state transitions, and diminishing of the OCP-dependent non-photochemical PBS quenching. In essence, even such a minute mutation of the PBS polypeptide component, like the PB-loop deletion, becomes important for the concerted function of the photosynthetic apparatus.

Thermostability of photosystem I trimers and monomers from the cyanobacterium Thermosynechococcus elongatus.

The performance of solar energy conversion into alternative energy sources in artificial systems highly depends on the thermostability of photosystem I (PSI) complexes Terasaki et al. (2007), Iwuchukwu et al. (2010), Kothe et al. (2013) . To assess the thermostability of PSI complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus heating induced perturbations on the level of secondary structure of the proteins were studied. Changes were monitored by Fourier transform infrared (FT-IR) spectra in the mid-IR region upon slow heating (1°C per minute) of samples in D2O phosphate buffer (pD 7.4) from 20°C to 100°C. These spectra showed distinct changes in the Amide I region of PSI complexes as a function of the rising temperature. Absorbance at the Amide I maximum of PSI monomers (centered around 1653cm-1), gradually dropped in two temperature intervals, i.e. 60-75 and 80-90°C. In contrast, absorbance at the Amide I maximum of PSI trimers (around 1656cm-1) dropped only in one temperature interval 80-95°C. The thermal profile of the spectral shift of α-helices bands in the region 1656-1642cm-1 confirms the same two temperature intervals for PSI monomers and only one interval for trimers. Apparently, the observed absorbance changes at the Amide I maximum during heating of PSI monomers and trimers are caused by deformation and unfolding of α-helices. The absence of absorbance changes in the interval of 20-65°C in PSI trimers is probably caused by a greater stability of protein secondary structure as compared to that in monomers. Upon heating above 80°C a large part of α-helices both in trimers and monomers converts to unordered and aggregated structures. Spectral changes of PSI trimers and monomers heated up to 100°C are irreversible due to protein denaturation and non-specific aggregation of complexes leading to new absorption bands at 1618-1620cm-1. We propose that monomers shield the denaturation sensitive sides at the monomer/monomer interface within a trimer, making the oligomeric structure more stable against thermal stress.

Protein-protein interactions in carotenoid triggered quenching of phycobilisome fluorescence in Synechocystis sp. PCC 6803.

An inquiry into the effect of temperature on carotenoid triggered quenching of phycobilisome (PBS) fluorescence in a photosystem II-deficient mutant of Synechocystis sp. results in identification of two temperature-dependent processes: one is responsible for the quenching rate, and one determines the yield of PBS fluorescence. Non-Arrhenius behavior of the light-on quenching rate suggests that carotenoid-absorbed light triggers a process that bears a strong resemblance to soluble protein folding, showing temperature-dependent enthalpy of activated complex formation. The response of PBS fluorescence yield to hydration changing additives and to passing of the membrane lipid phase transition point indicates that the pool size of PBSs subject to quenching depends on the state of some membrane component.