title: Biophysical modeling of in vitro and in vivo processes underlying regulated photoprotective mechanism in cyanobacteria.
abstract: 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 ~10-5M, 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.
authors.affiliation: Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1/2, Moscow, Russia, 119991. shirshin@lid.phys.msu.ru.
authors.forename: Evgeny A
authors.initials: EA
authors.lastname: Shirshin
authors.affiliation: Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1/2, Moscow, Russia, 119991.
authors.forename: Elena E
authors.initials: EE
authors.lastname: Nikonova
authors.affiliation: Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1/2, Moscow, Russia, 119991.|Department of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ, 08901, USA.
authors.forename: Fedor I
authors.initials: FI
authors.lastname: Kuzminov
authors.affiliation: A.N. Bach Institute of Biochemistry, Research Center of Biotechnology of the Russian Academy of Sciences, Moscow, Russia, 119071.|Department of Biology, M.V. Lomonosov Moscow State University, Leninskie Gory 1/12, Moscow, Russia, 119991.
authors.forename: Nikolai N
authors.initials: NN
authors.lastname: Sluchanko
authors.affiliation: Department of Biology, M.V. Lomonosov Moscow State University, Leninskie Gory 1/12, Moscow, Russia, 119991.
authors.forename: Irina V
authors.initials: IV
authors.lastname: Elanskaya
authors.affiliation: Department of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ, 08901, USA.
authors.forename: Maxim Y
authors.initials: MY
authors.lastname: Gorbunov
authors.affiliation: Department of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1/2, Moscow, Russia, 119991.
authors.forename: Victor V
authors.initials: VV
authors.lastname: Fadeev
authors.affiliation: Institute of Chemistry PC 14, Technical University of Berlin, Straße des 17. Juni 135, 10623, Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Friedrich
authors.affiliation: Department of Biology, M.V. Lomonosov Moscow State University, Leninskie Gory 1/12, Moscow, Russia, 119991.
authors.forename: Eugene G
authors.initials: EG
authors.lastname: Maksimov
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D055592:Biophysical Phenomena / Q000528:radiation effects; D002338:Carotenoids / Q000378:metabolism; D000458:Cyanobacteria / Q000378:metabolism* / Q000528:radiation effects*; D005453:Fluorescence; D007700:Kinetics; D008027:Light; D055668:Photochemical Processes / Q000528:radiation effects; D046939:Synechocystis / Q000378:metabolism / Q000528:radiation effects
chemical_list: D001426:Bacterial Proteins; D002338:Carotenoids
keywords: Biophysical model; Cyanobacteria; Fluorescence; Fluorescence recovery protein; Non-photochemical quenching; Orange carotenoid protein