Flavodiiron Protein Flv2/Flv4-Related Photoprotective Mechanism Dissipates Excitation Pressure of PSII in Cooperation with Phycobilisomes in Cyanobacteria      

Abstract

(A.R., I.V.) Oxygenic photosynthesis evolved with cyanobacteria, the ancestors of plant chloroplasts. The highly oxidizing chemistry of water splitting required concomitant evolution of efficient photoprotection mechanisms to safeguard the photosynthetic machinery. The role of flavodiiron proteins (FDPs), originally called A-type flavoproteins or Flvs, in this context has only recently been appreciated. Cyanobacterial FDPs constitute a specific protein group that evolved to protect oxygenic photosynthesis. There are four FDPs in Synechocystis sp. PCC 6803 (Flv1 to Flv4). Two of them, Flv2 and Flv4, are encoded by an operon together with a Sll0218 protein.Their expression, tightly regulated by CO 2 levels, is also influenced by changes in light intensity. Here we describe the overexpression of the flv4-2 operon in Synechocystis sp. PCC 6803 and demonstrate that it results in improved photochemistry of PSII. The flv4-2/OE mutant is more resistant to photoinhibition of PSII and exhibits a more oxidized state of the plastoquinone pool and reduced production of singlet oxygen compared with control strains. Results of biophysical measurements indicate that the flv4-2 operon functions in an alternative electron transfer pathway from PSII, and thus alleviates PSII excitation pressure by channeling up to 30% of PSII-originated electrons. Furthermore, intact phycobilisomes are required for stable expression of the flv4-2 operon genes and for the Flv2/Flv4 heterodimer-mediated electron transfer mechanism. The latter operates in photoprotection in a complementary way with the orange carotenoid protein-related nonphotochemical quenching. Expression of the flv4-2 operon and exchange of the D1 forms in PSII centers upon light stress, on the contrary, are mutually exclusive photoprotection strategies among cyanobacteria.Photosynthetic light reactions are evolutionarily highly conserved among oxygenic photosynthetic organisms from cyanobacteria to higher plants. Because of dangerous chemistry of the water splitting reactions, oxygenic photosynthesis produces reactive oxygen species (ROS) and other radicals that potentially could destroy the photosynthetic machinery. To avoid permanent damage, all oxygenic photosynthetic organisms are equipped with an array of various photoprotective and regulatory mechanisms. Accumulating evidence on these regulatory mechanisms has revealed vast evolutionary differences between organisms performing oxygenic photosynthesis.Photosynthetic organisms have a capacity to adjust to different light intensities and to changes in the availability of electron sinks, which depends largely on metabolic cues. When light or metabolic conditions change, photosystems can dissipate excess energy as heat in nonphotochemical energy dissipation processes in the light-harvesting antenna systems (for review, see Horton et al., 1996;Müller et al., 2001). Cyanobacteria have phycobilisomes (PBs) as light-harvesting antenna, which also participate in state transitions (for review, see van Thor et al., 1998;Mullineaux ...

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

“…PCC 6803. This inference is further supported by the lack of FLV2 and FLV4 homologs in the genome of S. elongatus PCC 7942 (Bersanini et al, 2014). In addition, we found oxygen-reducing activities of recombinant glutathione S-transferase (GST)-FLV4 fusion protein, similar to those of recombinant FLV3 protein (Vicente et al, 2002).…”

“…PCC 6803 (Hayashi et al, 2014). Furthermore, the genes encoding FLV2 and FLV4 are absent from the genome of S. elongatus PCC 7942 (Bersanini et al, 2014), while flv1 and flv3 are present (Synpcc7942_1810, ortholog of flv1; Synpcc7942_1809, ortholog of flv3). We hypothesized that FLV2 and FLV4 are involved in the AEF under CO 2 -limited photosynthesis and constructed mutants deficient in flv2 and flv4 (Dflv2 and Dflv4, respectively; Supplemental Figs.…”

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

“…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. FLV2/4 is known to interact with PSII and phycobilisomes (Bersanini et al, 2014), so in S. 6803, it may have multiple functions to alleviate photoinhibition under low CO 2 .…”

“…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. FLV2/4 is known to interact with PSII and phycobilisomes (Bersanini et al, 2014), so in S. 6803, it may have multiple functions to alleviate photoinhibition under low CO 2 .…”

Flavodiiron Proteins Promote Fast and Transient O2 Photoreduction in Chlamydomonas

“…However, a functional Flv3 homo-oligomer may also be formed, as deduced from the overproduction of Flv3 in Synechocystis (Mustila et al, 2016). On the other hand, Flv2 and Flv4 protect PSII by a different mechanism and using a yetuncharacterized electron acceptor (Zhang et al, 2012;Bersanini et al, 2014). From our data, we conclude that C. reinhardtii FlvA and FlvB are involved in O 2 photoreduction at the PSI acceptor side and therefore likely functions in a similar manner as cyanobacterial Flv1 and Flv3.…”