Spectroscopic and Functional Characterizations of Cyanobacterium Synechocystis PCC 6803 Mutants on and near the Heme Axial Ligand of Cytochrome b559 in Photosystem II

Abstract

2 is one of the essential components of the photosystem II (PSII) in higher plants, green algae, and cyanobacteria (1-5). cyt b 559 is a heme-bridged heterodimer protein that is composed of one ␣-and one ␤-subunit (encoded by the psbE and psbF genes) of 9 and 4 kDa, respectively. Each subunit provides a His ligand (His-22 residue of ␣-or ␤-subunit of cyt b 559 ) for the non-covalently bound heme. In addition, cyt b 559 exhibits different redox potential forms; a high potential form with a midpoint redox potential around ϩ400 mV, an intermediate potential form around ϩ200 -250 mV, and a low potential form with a midpoint redox potential about ϩ50 -100 mV (Refs. 5-9 and references therein). Several previous studies have proposed that cyt b 559 participates in secondary electron transfer pathways, which protect PSII from photoinhibition (5, 10 -12). In these models the high potential form of cyt b 559 might donate its electron to reduce highly oxidized chlorophyll radical species generated in PSII reaction centers under the donor-side photoinhibitory conditions. On the other hand, cyt b 559 might accept an electron from the acceptor side of PSII (Q B , PQ, or pheophytin (primary pheophitin a electron acceptor)) from generating damaging singlet oxygen species under the acceptor-side photoinhibitory conditions (13-15). In addition, a novel quinonebinding site (Q C ) was identified in proximity to cyt b 559 in the new 2.9Å PSII crystal structure (4). The occupancy of this Q C site has been proposed to modulate the redox equilibration between cyt b 559 and the PQ pool (16,17) or to involve in the exchange of PQ on the Q B site from the pool (4). Despite the recent progress in understanding the structure and function of PSII, the exact function of cyt b 559 in PSII remains unclear.Prior mutagenesis studies with Synechocystis sp. PCC6803, Chlamydomonas reinhardtii, or Nicotiana tabacum showed no stable PSII reaction centers assembled in the absence of either cyt b 559 subunit (18 -24). In addition, an early site-directed mutagenesis study with Synechocystis 6803 showed that substituting either of the heme axial ligands (His-22 of the ␣-subunit or His-22 of the ␤-subunit) with Leu severely diminished the assembly or stability of PSII (23). Furthermore, another sitedirected mutagenesis study involving C. reinhardtii showed that the H22Y and H22M mutants of the cyt b 559 ␣ subunit accumulated 10 -15% of the PSII (compared with wild-type cells) and contained a disrupted heme pocket, whereas still retaining significant amounts of O 2 evolution activity (24). This

The Roles of Cytochrome b559 in Assembly and Photoprotection of Photosystem II Revealed by Site-Directed Mutagenesis Studies

“…Furthermore, H22Kα and Y18Sα Cyt b 559 mutants in a D1-D170A genetic background that prevented assembly of the Mn cluster showed almost completely abolished accumulation of PSII even under normal-growth-light conditions. The data support an important redox role of Cyt b 559 in protecting PSII under donor-side photoinhibition conditions ( Hung et al, 2010 ).…”

“…PCC 6803) with Leu, Met, Glu, Gln, Tyr, Lys, Arg, or Cys abolished the photoautotrophic growth and severely diminished the assembly or stability of PSII in the mutant cells, except for H22Kα mutant cells, which were able to grow photoautotrophically and accumulated stable PSII reaction centers (∼81% as compared with wild-type cells; Pakrasi et al, 1991 ; Hung et al, 2007 , 2010 ). Electron paramagnetic resonance results indicated the displacement of one of the two axial ligands to the heme of Cyt b 559 in H22Kα mutant reaction centers, at least in isolated reaction centers ( Hung et al, 2010 ). In addition, H22Kα and Y18Sα (corresponding to Y19Sα in T. elongatus ) in mutant PSII core complexes contained predominately the LP form of Cyt b 559 .…”

“…Spectroscopic and functional characterization of the H22Kα and Y18Sα Cyt b 559 mutant cells of Synechocystis sp. PCC 6803 showed that both mutants have functional PSII and exhibited the normal period-four oscillation in oxygen yield ( Hung et al, 2010 ). However, both mutants were more susceptible to photoinhibition than the wild type under high-light conditions.…”

Diurnal Changes in Transcript and Metabolite Levels during the Iron Deficiency Response of Rice

“…Iron (Fe) is utilized in a variety of essential biological processes such as the respiratory (Park et al 1996) and photosynthetic (Hung et al 2010) electron-transport chain reactions (Connolly and Guerinot 2002; Reddy 2006), due to its stable redox properties, specifically an ability to donate and accept electrons between both Fe +2 (ferrous) and Fe +3 (ferric) redox states. Although Fe is the fourth most abundant element in the earth’s crust, it is not readily available to soil growing plants under aerobic conditions due to its tendency to form insoluble Fe 3+ precipitates (Kim and Guerinot 2007).…”

Az Acaryochloris marina cianobaktérium kettes fotokémiai rendszerének D1, D2 és citokróm b559 alegységeit, valamint a Hox hidrogenázt kódoló gének kifejeződésének vizsgálata

“…A hemcsoport kötésért felelős hisztidin (His23) mindkét PsbE szekvenciában konzervált, azonban a PsbE' számos aminosav mutációt tartalmaz a hem-kötő His közvetlen közelében. Ilyen például a Tyr19 mutációja, amely erősen fényérzékeny fenotípus kialakulásához vezet (61). A PsbE'-ot kódoló psbE1 rendkívül alacsony relatív transzkript szintet mutat, és a psbE1 gént érintő transzkripciós változást a dolgozatban vizsgált körülmények egyikében sem tapasztaltunk (nem bemutatott adat), ami a számos aminosav mutációval együtt a psbE1 pszeudogén voltára utal.…”