FLAVODIIRON2 and FLAVODIIRON4 Proteins Mediate an Oxygen-Dependent Alternative Electron Flow in Synechocystis sp. PCC 6803 under CO2-Limited Conditions  

Abstract

(A.M., C.M.) This study aims to elucidate the molecular mechanism of an alternative electron flow (AEF) functioning under suppressed (CO 2 -limited) photosynthesis in the cyanobacterium Synechocystis sp. PCC 6803. Photosynthetic linear electron flow, evaluated as the quantum yield of photosystem II [Y(II)], reaches a maximum shortly after the onset of actinic illumination. Thereafter, Y(II) transiently decreases concomitantly with a decrease in the photosynthetic oxygen evolution rate and then recovers to a rate that is close to the initial maximum. These results show that CO 2 limitation suppresses photosynthesis and induces AEF. In contrast to the wild type, Synechocystis sp. PCC 6803 mutants deficient in the genes encoding FLAVODIIRON2 (FLV2) and FLV4 proteins show no recovery of Y(II) after prolonged illumination. However, Synechocystis sp. PCC 6803 mutants deficient in genes encoding proteins functioning in photorespiration show AEF activity similar to the wild type. In contrast to Synechocystis sp. PCC 6803, the cyanobacterium Synechococcus elongatus PCC 7942 has no FLV proteins with high homology to FLV2 and FLV4 in Synechocystis sp. PCC 6803. This lack of FLV2/4 may explain why AEF is not induced under CO 2 -limited photosynthesis in S. elongatus PCC 7942. As the glutathione S-transferase fusion protein overexpressed in Escherichia coli exhibits NADHdependent oxygen reduction to water, we suggest that FLV2 and FLV4 mediate oxygen-dependent AEF in Synechocystis sp. PCC 6803 when electron acceptors such as CO 2 are not available.In photosynthesis, photon energy absorbed by PSI and PSII in thylakoid membranes oxidizes the reaction center chlorophylls (Chls), P700 in PSI and P680 in PSII, and drives the photosynthetic electron transport (PET) system. In PSII, water is oxidized to oxygen as the oxidized P680 accepts electrons from water. These electrons then reduce the cytochrome b 6 /f complex through plastoquinone (PQ) in the thylakoid membranes. Photooxidized P700 in PSI accepts electrons from the reduced cytochrome b 6 /f complex through plastocyanin or cytochrome c 6 . Electrons released in the photooxidation of P700 are used to produce NADPH through ferredoxin and ferredoxin NADP + reductase. Thus, electrons flow from water to NADPH in the so-called photosynthetic linear electron flow (LEF). Importantly, LEF induces a proton gradient across the thylakoid membranes, which provides the driving force for ATP production by ATP synthases in the thylakoid membranes. NADPH and ATP serve as chemical energy donors in the photosynthetic carbon reduction cycle (Calvin cycle).It recently has been proposed that, in cyanobacteria, the photorespiratory carbon oxidation cycle (photorespiration) functions simultaneously with the Calvin cycle to recover carbon for the regeneration of ribulose-1,5-bisphosphate, one of the substrates of Rubisco (Hagemann et al., 2013). Rubisco catalyzes the primary reactions of carbon reduction as well as oxidation cycles. However, the presence of a specific carbon concentration mechan...

Oxidation of P700 in Photosystem I Is Essential for the Growth of Cyanobacteria

“…S6). These data suggest that FLV2/4 mediates an oxygen-dependent AEF on the acceptor side of PSI (Hayashi et al, 2014;Shimakawa et al, 2015), as does FLV1/3 (Helman et al, 2003), since both FLV sets have similar primary structures (Fujisawa et al, 2014) and enzymatic characteristics of recombinant proteins (Vicente et al, 2002;Shimakawa et al, 2015). However, we cannot exclude the possibility that the relief of excitation pressure at PSII by FLV2/4 (Bersanini et al, 2014) provides an enhancement of Y(I) during CO 2 -limited photosynthesis in S. 6803.…”

“…Y(I) and Y(II) were restored when CO 2 was added in the form of NaHCO 3 ( Fig. 3A; Hayashi et al, 2014;Shimakawa et al, 2015). The P700 redox state also responded to CO 2 limitation.…”

“…Therefore, reduced activity of NAD(P)H dehydrogenasemediated cyclic electron flow (Ohkawa et al, 2000) is not linked to the ratio of Y(I) to Y(II). Moreover, cyclic electron transport rates in S. 6803 and S. 7002 are negligible relative to their photosynthetic linear and Shimakawa et al (2015).…”

“…Unlike FLV1/3, FLV2/4 is induced only under low CO 2 (Zhang et al, 2009) and mediates an oxygen-dependent alternative electron flow (AEF; Shimakawa et al, 2015). In S. 6803, FLV-dependent electron fluxes are coupled to photosynthesis and should alleviate electron overaccumulation in PSI (Helman et al, 2003(Helman et al, , 2005Allahverdiyeva et al, 2013;Shimakawa et al, 2015). Therefore, FLV is expected to contribute to P700 oxidation.…”

“…FLV1 and FLV3 (FLV1/3) function as a heterodimer and catalyze the reduction of oxygen to water on the acceptor side of PSI using NAD(P)H as electron donors (Vicente et al, 2002;Helman et al, 2003;Allahverdiyeva et al, 2013). Unlike FLV1/3, FLV2/4 is induced only under low CO 2 (Zhang et al, 2009) and mediates an oxygen-dependent alternative electron flow (AEF; Shimakawa et al, 2015). In S. 6803, FLV-dependent electron fluxes are coupled to photosynthesis and should alleviate electron overaccumulation in PSI (Helman et al, 2003(Helman et al, , 2005Allahverdiyeva et al, 2013;Shimakawa et al, 2015).…”