Revisiting Cyanobacterial State Transitions: the role of cytb6f and new questions arising from the ΔApcD Synechococcus elongatus mutant

CALZADILLA P.I.; MUZZOPAPPA, FERNANDO; ZHAN, JIAO; SÉTIF, PIERRE; KIRILOVSKY, DIANA

Uppsala

Conferencia; EPS1 - 1st European Conference for Photosynthesis Research; 2018

Photosynthetic organisms adapt to changing light environment through different mechanisms. ?Statetransitions? is one of these processes, controlling the relative extent of energy transfer from antennae tophotosystems. In cyanobacteria, light is absorbed by large protein complexes on the surface of thethylakoid membrane called phycobilisomes (PBS), constituted by phycobiliproteins organized in a corefrom which rods radiate. Within the core, apcD, apcF and apcE are involved in energy transfer tophotosystems. The ΔapcD and DApcF mutants of Synechocystis PCC 6803 and Synechococcus PCC7002 lack state transitions. In this process, two ?States? are defined: State II is characterized by relativelow Photosystem II (PSII) to Photosystem I (PSI) fluorescence ratio, and State I by high PSII to PSIfluorescence ratio. The transition from one state to the other is triggered by changes in the redox state ofthe plastoquinone (PQ) pool. In plants and algae, the redox sensor of the PQ pool is the cytochrome b6f(cyt b6f), which interacts with a kinase that specifically phosphorylates the major membrane chlorophyllantenna, the Light Harvesting II (Wollman 2001). It was suggested that cyt b6f has also a role incyanobacterial state transition (Huang et al, 2003; Mao et al, 2002). This proposition was based onresults showing that DBMIB inhibits State I transition even in the presence of PQ pool electron acceptors(DMBQ and PPBQ), which oxidize the PQ pool. In this study we tested the effect of these chemicals onthe state transition of Synechococcus elongatus (PCC7492), the effect of different kinases inhibitors andcharacterized its ΔApcD and ΔApcF mutants. Comparison with Synechocystis WT and ΔApcD were alsodone. Our results show that the effect of DBMIB, in the presence of DMBQ, is due to alterations in theredox state of the PQ pool, and could not be directly linked to cyt b6f. In this sense, the triggering signalfor state transitions in cyanobacteria remains to be elucidated. In addition, no effect was observed in thetreatments with the kinases inhibitors, suggesting that phosphorylation is not involved in this process inthe studied strain. Moreover, we (unexpectedly) found that the ΔApcD and DApcF S. elongatus mutantswere able to perform state transitions. Large changes in Chl PSII related fluorescence were observedbetween State I and II, suggesting that in this strain the fluorescence coming from the membranes ismore relevant for state transitions than the fluorescence coming from PBS. Altogether, the role of ApcDand ApcF seem to be different between S. elongatus and Synechocystis.