Biophysical modeling of in vitro and in vivo processes underlying regulated photoprotective mechanism in cyanobacteria
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Non-photochemical quenching (NPQ) is a mechanism responsible for high light tolerance in photosynthetic organisms. In cyanobacteria, NPQ is realized by the interplay between light-harvesting complexes, phycobilisomes (PBs), a light sensor and effector of NPQ, the photoactive orange carotenoid protein (OCP), and the fluorescence recovery protein (FRP). Here, we introduced a biophysical model, which takes into account the whole spectrum of interactions between PBs, OCP, and FRP and describes the experimental PBs fluorescence kinetics, unraveling interaction rate constants between the components involved and their relative concentrations in the cell. We took benefit from the possibility to reconstruct the photoprotection mechanism and its parts in vitro, where most of the parameters could be varied, to develop the model and then applied it to describe the NPQ kinetics in the Synechocystis sp. PCC 6803 mutant lacking photosystems. Our analyses revealed that while an excess of the OCP over PBs is required to obtain substantial PBs fluorescence quenching in vitro, in vivo the OCP/PBs ratio is less than unity, due to higher local concentration of PBs, which was estimated as ~10−5 M, compared to in vitro experiments. The analysis of PBs fluorescence recovery on the basis of the generalized model of enzymatic catalysis resulted in determination of the FRP concentration in vivo close to 10% of the OCP concentration. Finally, the possible role of the FRP oligomeric state alteration in the kinetics of PBs fluorescence was shown. This paper provides the most comprehensive model of the OCP-induced PBs fluorescence quenching to date and the results are important for better understanding of the regulatory molecular mechanisms underlying NPQ in cyanobacteria.
KeywordsCyanobacteria Non-photochemical quenching Orange carotenoid protein Fluorescence recovery protein Biophysical model Fluorescence
The work was supported by the Russian Foundation for Basic Research (Grant No. 16-05-01110), the German Ministry for Education and Research (to T.F.; WTZ-RUS grant 01DJ15007), the Russian Science Foundation (grant №14-17-00451), and the German Research Foundation—Cluster of Excellence “Unifying Concepts in Catalysis” (to T.F.). E.G.M. thanks the Russian Foundation for Basic Research (project No. 15-04-01930A), the Russian Ministry of Education and Science (project MK-5949.2015.4), the Dynasty Foundation Fellowship, RFBR, and Moscow City Government according to the research project No. 15-34-70007 «mol_а_mos» for partial support of this work. N.N.S. was supported by a scholarship from the President of Russian Federation (SP-367.2016.4). M.Y.G. was supported by NASA Ocean Biology and Biogeochemistry Program (Grant NNX16AT54G). We thank Kevin Wyman for comments on the manuscript.
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