Ciprian Chis, Iuliana-Cristina Simeoni, Oana Sicora and Cosmin Ionel Sicora New Insights in PSII Electron Transport Chain in Cyanobacterium Synechococcus sp. PCC 7002 Summary: The structure of the photosynthetic machinery in cyanobacteria is highly conserved, as well as in green algae and higher plants. The core proteins of photosystem II (PSII), D1 and D2, bind all the redox-active components involved in electron transfer of PSII. D1 protein is one of the main sites of damage by a variety of environmental factors, requiring its replacement, whereas most of the other PSII subunits remain ordinarily undamaged. The D1 protein family from cyanobacteria contains members with different functionality as an adaptation to different environmental conditions. There are members of the protein family involved in adaptation to high-light conditions, others to UV-B stress and more recently were discovered members induced in low oxygen or micro-aerobic conditions hinting about a role these D1 form play in cellular adaptation to above mentioned conditions. In this study we used a Synechococcus sp. PCC 7002 mutant that has an inhibited psbA gene encoding D1’ isoform in comparison with the wild type, in order to better understand the role of this D1 protein isoform under a range of environmental factors (UVB, high-light, micro-aerobic conditions). The standard growth conditions for this strain were: light irradiance of 50 µE x m -2 x s -1, and 38°C. During the high-light experiments Synechococcus sp. PCC 7002 shows a change in the decay of the fluorescence curve, not seen previously in other species. In our experiments we try to understand the nature and origin of these changes in PSII function in this cyanobacterium in an effort to gain more insight into the mechanisms of cyanobacterial photosynthetic electron transport. Ministry of National Education, Biological Research Center Jibou, Romania Introduction: The D1 protein family from cyanobacteria contains members with different functionality as an adaptation to different environmental conditions. There are members of the protein family involved in adaptation to high-light conditions, others to UV-B stress and more recently were discovered members induced In low oxygen or micro-aerobic conditions hinting about a role these D1 form play in cellular adaptation to above mentioned conditions. In Synechococcus sp. PCC 7002 we have 3 psbA genes encoding different isoforms of D1 protein. Materials and Methods Synechococcus sp. PCC 7002 wild type (WT) was obtained from Pasteur Culture Collection. The mutant of Synechococcus sp. PCC 7002, ∆A2164, was a kind gift from Dr. Yvonne Zilliges from the group of Professor Holger Dau from Freie Universitat Berlin. Both strains were grown at 38ºC and 50μmol photons m -2 s -1 light, until 6μg chl ml -1. High-light stress was applied for 120 minutes, followed by the return to the initial light conditions for a recovery time of 60 minutes. To another culture, the initial light conditions were replaced by UV-B for 120 minutes, and finally the initial light conditions were returned for 60 minutes. Cells were also grown in air, replaced with N 2 by bubbling for 120 minutes, in order to induce anaerobic conditions then the aerobic conditions were returned by bubbling with air for 60 minutes. The fluorescence measurements were made with an FL3500 Fluorometer from Photon Systems Instruments, we used a Q A - reoxidation protocol. Finally, data were processed in the Origin.8 program, to each sample was applied a Joliot correction before normalisation. Results Conclusions: Based on protein sequence, Synechococcus sp. PCC 7002 has a potential D1’ isoform candidate. Under steady state growth conditions the mutant shows an accelerated Q A to Q B transfer as compared to wild type that is particularly evident at 38ºC. The mutant cells also show a higher chlorophyll to phycobilisomes ratio suggesting a better light harvesting capacity. No statistically relevant difference was observed between mutant and wild type during micro-aerobic and UVB treated cells. During High Light stress the mutant is capable to recover better than the wild type after 120 minutes of applied stress, both in the presence and absence of DCMU. An increased amount of total D1 protein was detected during the recovery period, typical for an after stress cellular reaction. Figure 1: Alignment of the 3 protein sequences encoded by the psbA gene family in Synechococcus sp. PCC Positions characteristic for D1’ protein were marked with red arrows. Figure 2: Whole cell absorbance spectrum of Synechococcus sp. PCC 7002 wild type and ∆A2164 mutant strain, grown at 38ºC. Figure 3: Q A reoxidation measured by fluorescence decay in presence and absence of DCMU on WT and ∆A2164 cells grown at 30ºC (a) and 38ºC (b). Figure 4: The effect of micro-aerobic conditions induced by N 2 bubbling for 120 minutes and subsequent recovery on the acceptor and donor side of PSII in WT and ∆A2164 strain (a).Panel (b)shows the evolution of fluorescence amplitude during treatment and recovery in the presence and absence of DCMU. Figure 5: The effect of UVB treatment for 120 minutes and subsequent recovery on the acceptor and donor side of PSII in WT and ∆A2164 strain (a).Panel (b)shows the evolution of fluorescence amplitude during treatment and recovery in the presence and absence of DCMU. Figure 6: The effect of High Light treatment for 120 minutes and subsequent recovery on the acceptor and donor side of PSII in WT and ∆A2164 strain (a).Panel (b)shows the evolution of fluorescence amplitude during treatment and recovery in the presence and absence of DCMU. a)b) c) Figure 7: Detection of D1 protein using a global D1 antibody in Synechococcus sp. PCC 7002 cells during micro-aerobic treatment (a) UVB treatment (b) and High light treatment (c) Acknowledgement: This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE