Identification of the special pair of photosystem II in a chlorophyll d-dominated cyanobacterium

Abstract

The composition of photosystem II (PSII) in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017 was investigated to enhance the general understanding of the energetics of the PSII reaction center. We first purified photochemically active complexes consisting of a 47-kDa Chl protein (CP47), CP43' (PcbC), D1, D2, cytochrome b(559), PsbI, and a small polypeptide. The pigment composition per two pheophytin (Phe) a molecules was 55 +/- 7 Chl d, 3.0 +/- 0.4 Chl a, 17 +/- 3 alpha-carotene, and 1.4 +/- 0.2 plastoquinone-9. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FTIR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. The primary electron acceptor was shown by photoaccumulation to be Phe a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Taking into account the reported downward shift in the potential of the special pair of photosystem I (P740) in A. marina, our findings lend support to the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system.

Fig. 1.

Scheme for electron transfer and cofactor organization of PSII RC. Arrows show the direction of electron transfer. Pigments located on the D1 protein are shown with a subscript D1, and those on the D2 protein are shown with a subscript D2. P_D1 and P_D2, the special pair; Chl_D1 and Chl_D2, the accessory Chl; Phe_D1, the primary electron acceptor, pheophytin; Phe_D2, pheophytin; Chlz_D1 and Chlz_D2, peripheral Chl; Q_A, the secondary electron acceptor; Y_Z, secondary electron donor; Y_D, tyrosine on the D2 protein.

Fig. 2.

SDS/PAGE and Western blot analysis of PSII complexes isolated from A. marina. Lane 1, molecular marker; lane 2, thylakoid membranes; lane 3, partially purified PSII preparations after DEAE Toyopearl chromatography; lane 4, purified PSII complexes; lane 5, CP43′; lane 6, PSII RC isolated from spinach; lane 7, PSII core complex isolated from Synechocystis; lane 8, Western blot of purified PSII complex (lane 4) with anti-D1; lanes 9–11, Western blot with anti-PsaA/B; lane 9, thylakoid membranes (lane 2); lane 10, partially purified PSII (lane 3); lane 11, purified PSII complex (lane 4). Individual marks represent CP47 (filled circles), D2 (open circles), D1 (asterisks), CP43′ (filled squares), cyt b₅₅₉ α-subunit (open squares), PsbI (open triangles), and cyt b₅₅₉ β-subunit (filled triangles).

Fig. 3.

Absorption and fluorescence spectra of purified PSII complexes. (A) Absorption spectrum at 283 K. The second derivative spectrum is shown in the upper part, after inversion. (B) Fluorescence excitation spectra at 77 K monitored at 675 nm (solid line) and 679 nm (dotted line). (Inset) Fluorescence spectrum at 77 K upon excitation at 435 nm.

Fig. 4.

Identification of the special pair of PSII complexes. (A) Identification by the reversible absorption change of the sample in the Qy region at 298 K. Difference spectrum during blue light illumination is shown. (B and C) Identification by FTIR spectroscopy. Shown are light-induced cation-minus-neutral FTIR difference spectra of the special pair in PSII core complexes of A. marina (line a) and Synechocystis (line b) and in PSII membranes of spinach (line c). Separate overlays are shown for 1,800–1,100 cm⁻¹, the region involving CO stretching and chlorin ring vibrations (B), and 2,770–2,660 cm⁻¹, the region involving the CH stretching vibration of the 3-formyl group (C). Spectra were recorded at 250 K.

Fig. 5.

Changes in the redox potential of Phe a in A. marina. (A) Light-induced absorption difference spectra in the Qx region of Phe a. Shown are Q_A-depleted PSII complexes from A. marina (solid black line) and PSII complexes with sodium dithionite (solid gray line). The dotted line shows the PSII core from Synechocystis. For the solid black line and the dotted line, the scale on the left applies. (B Left) Suppression of DF in the presence of sodium dithionite. The excitation wavelength was 400 nm. Fluorescence decay at 685 nm was measured at 77 K in the absence (solid line) or presence (dashed line) of 4 mM sodium dithionite. (B Right) Wavelength-dependent fluorescence decay curves of DF at 77 K in the absence of dithionite.