Impair Of Photosystem II (PSII) Function During Day And Night Cycles In Cyanobacterium Cyanothece sp. ATCC Is A Two Phase Process Partly Dependent To Protein Synthesis Summary : Cyanobacteria have highly conserved photosynthetic machinery. The D1 protein, the core protein of Photosystem II, is an indispensable component of oxygenic photosynthesis. The D1 protein is encoded by a psbA gene family, whose members are differentially expressed in different environmental conditions. The D1 protein is expressed by 5 psbA genes, encoding 4 isoforms of the protein. Recent publications postulate the presence of another class of cyanobacterial D1 proteins, D1 rogue class, which blocks the PSII during the night and allows the N 2 fixation. Cyanothece sp. ATCC is a unicellular, aerobic, diazotrophic cyanobacterium which fixes N 2 in the dark. We measured the PSII function at cells grown in continuous light and in 12 hours light-dark cycles. We observed changes in PSII function during night, a deceleration of the electron transfer between Q A and Q B in the acceptor side of the electron transport chain and also a possible effect of the water oxidation complex in the donor side of the electron transport chain. We used a protein synthesis inhibitor to discriminate between the temporary impairment of PSII and more profound changes caused by the exchange of the D1 proteins. Our results show that the inhibitory process recorded during night time is a combination of the two processes with a fast initial inhibition that is not protein dependent and a later phase that requires synthesis of new proteins. Iuliana Chis 1, Oana Sicora 1, Ciprian Chis 1, Mara Ana Naghi 1, Carmel Dalton 1,2, Cosmin Sicora 1 1 Biological Research Center Jibou, Romania 2 Western University “Vasile Goldis”, Arad, Romania Materials and methods: Cyanothece sp. ATCC was obtained from the Biological Research Institute from Cluj Napoca, Romania. The strain was grown at 30ºC and 50μmol photons m -2 s -1 light, until a chlorophyll concentration of 6 μg chl ml -1. In order to investigate the changes in PS II function during 12 h light/12 h dark cycles, we designed the following experiments: we performed flash fluorescence measurements on Cyanothece sp. ATCC 51142, every 60 minutes during 26 h, both in the absence and presence of DCMU (3-(3,4-Dichlorophenyl)-1,1-dimethylurea). We also measured chlorophyll fluorescence in the presence of Lincomycin (300µg/ml), an inhibitor that blocks the synthesis of new proteins. As control, similar measurements were also performed on Synechocystis sp. PCC The fluorescence measurements were made with an FL3500 fluorometer from Photon Systems Instruments, we used a Q A recombination protocol, then the results were processed in the Origin.v8.0 program and for each sample a Joliot Correction was applied before plotting the graphs. Acknowledgement: This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE Conclusions: We observed changes in PSII function during night, a deceleration of the electron transfer between Q A and Q B in the acceptor side of PSII and also a possible effect of the water oxidation complex in the donor side. The effect is partly dependent on protein synthesis. Our results show that the inhibitory process recorded during night time is a combination of the two processes with a fast initial inhibition that is not protein dependent and a later phase that requires synthesis of new proteins. Figure 6: T2 parameter evolution on the acceptor side of PS II function during a 12h Light/12h Dark cycle in Cyanothece sp. ATCC 51142, in absence and presence of Lincomycin. Figure 8: Changes of acceptor side of PS II function during dark in Cyanothece sp. ATCC 51142, in absence and presence of Lincomycin. Figure 7: Changes in the number of PS II active centers during a 12h Light/12h Dark cycle in Cyanothece sp. ATCC in absence (a) and presence (b) of DCMU Figure 4: Changes of acceptor side of PS II function during Light/Dark cycle in Cyanothece sp. ATCC Figure 5: Changes of donor side of PS II function during Light/ Dark cycle in Cyanothece sp. ATCC Figure 2: Changes in the number of PS II active centers during a 12h Light/12h Dark cycle in Cyanothece sp. ATCC in absence (a) and presence (b) of DCMU Figure 1: Changes of acceptor and donor side of PS II function during light in Cyanothece sp. ATCC Figure 3: Changes of acceptor and donor side of PS II function during dark in Cyanothece sp. ATCC 51142