ESR Signal of the Iron−Sulfur Center F_Xand Its Function in the Homodimeric Reaction Center ofHeliobacterium modesticaldum^,

Abstract

Electron transfer in the membranes and the type I reaction center (RC) core protein complex isolated from Heliobacterium modesticaldum was studied by optical and ESR spectroscopy. The RC is a homodimer of PshA proteins. In the isolated membranes, illumination at 14 K led to accumulation of a stable ESR signal of the reduced iron-sulfur center F(B)(-) in the presence of dithiothreitol, and an additional 20 min illumination at 230 K induced the spin-interacting F(A)(-)/F(B)(-) signal at 14 K. During illumination at 5 K in the presence of dithionite, we detected a new transient signal with the following values: g(z)= 2.040, g(y)= 1.911, and g(x)= 1.896. The signal decayed rapidly with a 10 ms time constant after the flash excitation at 5 K and was attributed to the F(X)(-)-type center, although the signal shape was more symmetrical than that of F(X)(-) in photosystem I. In the purified RC core protein, laser excitation induced the absorption change of a special pair, P800. The flash-induced P800(+) signal recovered with a fast 2-5 ms time constant below 150 K, suggesting charge recombination with F(X)(-). Partial destruction of the RC core protein complex by a brief exposure to air increased the level of the P800(+)A(0)(-) state that gave a lifetime (t(1/2)) of 100 ns at 77 K. The reactions of F(X) and quinone were discussed on the basis of the three-dimensional structural model of RC that predicts the conserved F(X)-binding site and the quinone-binding site, which is more hydrophilic than that in the photosystem I RC.

The Genome of Heliobacterium modesticaldum , a Phototrophic Representative of the Firmicutes Containing the Simplest Photosynthetic Apparatus

“…It is known that rapid transfer occurs from the excited state of the primary electron donor, P798, to the primary acceptor, 8 1 -OH-Chl a (A 0 ), but the existence of a secondary quinone acceptor (A 1 ) has not been confirmed (41). PshA exists as a homodimer containing two cysteine residues per subunit that are believed to bind to an F X -like [4Fe-4S] cluster (26,38,41). In addition to pshA, a pshB gene (HM1_1462), which encodes an RC-associated [4Fe-4S]-binding ferredoxin, was confirmed to be present in the H. modesticaldum genome.…”

Modulation of fluorescence in Heliobacterium modesticaldum cells

“…An F x binding motif was first identified in the RC polypeptide of Heliobacillus mobilis (Liebl et al 1993), and the presence of the F x cluster was later confirmed spectroscopically (Heinnickel et al 2006;Miyamoto et al 2006). There is, however, some controversy as to whether the ground state spin is equal to 1/2 (Miyamoto et al 2006) or 3/2 (Heinnickel et al 2006). Two additional [4Fe-4S] clusters, F A and F B , can be reduced by photo-accumulation and distinguished by low-temperature EPR (Nitschke et al 1990).…”

“…A putative quinone-binding site in the PshA polypeptide can be predicted, by comparison with the A 1 sites of PsaA/PsaB, but it is should be more polar, due to the replacement of the Trp that p-stacks with phylloquinone in PS1 (Miyamoto et al 2006;Oh-oka 2007) by Arg. Menaquinone is the sole quinone that was found to exist in the membranes of Heliobacterium chlorum, consisting of *79% MK-9 and *15% MK-8 (Hiraishi 1989).…”

The bound iron–sulfur clusters of Type-I homodimeric reaction centers

“…No low-field resonances that could be attributed to an S = 3/2 ground spin state were observed. Curiously, the spectrum of F X was not observed when the F A and F B clusters were removed (Miyamoto et al 2006). …”

“…However, SDS-PAGE of a wide variety of purified HbRCs failed to reveal a suitable candidate. Moreover, the EPR and time-resolved optical signals characteristic of the F A and F B clusters were invariably absent in published HbRC preparations (Trost and Blankenship 1989;Fuller et al 1985;van de Meent et al 1990;Miyamoto et al 2006). Further complicating the issue was the absence of an fdx-like gene in or near the photosynthetic gene cluster, a region that contains many of the genes required to synthesize an active HbRC (Xiong et al 1998).…”