PsaE Is Required for in Vivo Cyclic Electron Flow around Photosystem I in the Cyanobacterium Synechococcus sp. PCC 7002

Abstract

Electron transfer rates to P700' have been determined in wildtype and three interposon mutants (psaE-, ndhf-, and psaE-ndhf-) of Synechococcus sp. PCC 7002. All three mutants grew significantly more slowly than wild type at low light intensities, and each failed to grow photoheterotrophically in the presence of 3-(3,4-dichloropheny1)-1,l-dimethylurea (DCMU) and a metabolizable carbon source. l h e kinetics of P700+ reduction were similar in the wild-type and mutant whole cells in the absence of DCMU. In the presence of DCMU, the P700+ reduction rate in the psaE mutant was significantly slower than in the wild type. In the presence of DCMU and potassium cyanide, added to inhibit the outflow of electrons through cytochrome oxidase, P700+ reduction rates increased for both the psaE-and ndhF-strains. The reduction rates for these two mutants were nonetheless slower than that observed for the wild-type strain. l h e further addition of methyl viologen caused the rate of P700+ reduction in the wild type to become as slow as that for the psaE mutant in the absence of methyl viologen. Civen the ability of methyl viologen to intercept electrons from the acceptor side of photosystem I, this response reveals a lesion in cyclic electron flow in the psaE mutant. In the presence of DCMU, the rate of P700' reduction in the psaE ndhf double mutant was very slow and nearly identical with that for the wild-type strain in the presence of 2,4-dibromo-3-methyl-6-isopropyl-p-benzoquinone, a condition under which physiological electron donation to P700+ should be completely inhibited. These results suggest that NdhF-and PsaE-dependent electron donation to P700+ occurs only via plastoquinone and/or cytochrome b6/f and indicate that there are three major electron sources for P700+ reduction in this cyanobacterium. We conclude that, although PsaE is not required for linear electron flow to NADP+, it is an essential component in the cyclic electron transport pathway around photosystem 1.

Structure and properties in solution of PsaD, an extrinsic polypeptide of photosystem I

“…The eluted fractions were analysed by SDS/PAGE and samples containing PsaD were concenthe structure of PsaD in solution differs from the native structure in situ, the results are relevant to the assembly of PsaD into PSI trated in an Amicon ultrafilter over a YM3 membrane. The yield of [ 13 C, 15 N]PsaD was 6 mg/l when cells were grown on minimal in vivo.…”

“…PsaE is also thought to play a role in ferredoxin reduction [12Ϫ14], as well as in cyclic to chemical free energy (around 40%). In cyanobacteria, it is known to contain two large, intrinsic membrane subunits (PsaA, electron flow around PSI [15]. Neither PsaD nor PsaE contain a redox cofactor.…”

“…1D (one-dimensional) 1 H spectra were acplasmid pET-3a/D were grown on M9 minimal medium contain-quired with 512 scans, a NOESY pulse sequence with presaturaing 50 µg/ml ampicillin, supplemented with vitamin mixture and tion of the solvent signal, and a spectral width of 8064.52 Hz. P1 trace metal solution [21] 15 N]PsaD a doubly labelled algal hydrolysate to suppress the water signal, with 64 complex points and a (1 g/l) and [ 13 C]glucose (4 g/l) were added to the medium. Cells spectral width of 1500 Hz in t 1 , and with 2048 real points and a were grown on a 10 ml Luria-Bertani culture overnight at 37°C.…”

Changes in the transthylakoid proton gradient are caused by the movement of phycobilisomes in the cyanobacterium Synechocystis sp. strain PCC 6803

“…A balanced distribution of the excitation energy absorbed by light-harvesting complexes to the complexes of photosystems I and II (PSI and II) is considered to be one of the most important factors for ensuring optimal photosynthetic efficiency [1][2][3]. In response to fluctuating light conditions, the photosynthetic machinery regulates the distribution of excitation energy between the 2 photosystems (PSs) [4]. This dynamic and rapid process of achieving energy balance is called "state transition" [5,6].…”

“…In response to fluctuating light conditions, the photosynthetic machinery regulates the distribution of excitation energy between the 2 photosystems (PSs) [4]. This dynamic and rapid process of achieving energy balance is called "state transition" [5,6].…”