Electron transfer components of wild-type and photosynthetic mutant strains of Scenedesmus obliquus D3

Cytochrome cM from synechocystis 6803. Detection in cells, expression in Escherichia coli, purification and physical characterization

Based on DNA sequence data a novel c-type cytochrome, cytochrome cM, has been predicted to exist in the cyanobacterium Synechocystis 6803. The precursor protein consists of 105 amino acids with a characteristic heme-binding motif and a hydrophobic domain located at the N-terminal end that is proposed to act as either a signal peptide or a membrane anchor. For the first time we report the detection of cytochrome cM in Synechocystis 6803 using Western blot analysis. The soluble portion cytochrome cM has been overexpressed in Escherichia coli in two forms, one with a poly histidine tag to facilitate purification and one without such a tag. The overexpressed protein has been purified and shown to bind heme, exhibiting an absorption peak in the Soret band near 416 nm and a peak in the alpha band at 550 nm. The extinction coefficient of cytochrome cM is 23.2 +/- 0.5 mM-1.cm-1 for the reduced minus oxidized alpha band peak (550-535 nm). The isoelectric point of cytochrome cM is 5.6 (without the histidine tag), which is significantly lower than the pI of 7.2 predicted from the amino acid sequence. The redox midpoint potential of cytochrome cM expressed in E. coli is 151 +/- 5 mV (pH 7.1), which is quite low compared to other c-type cytochromes in which a histidine and a methionine residue serve as the axial ligands to the heme. This work opens the way for determining the three-dimensional structure of cytochrome cM and investigating its function in cyanobacteria.

PHOTOSYNTHETIC CYTOCHROMES c IN CYANOBACTERIA, ALGAE, AND PLANTS

The cytochromes that function in photosynthesis in cyanobacteria, algae, and higher plants have, like the other photosynthetic catalysts, been largely conserved in their structure and function during evolution. Cyanobacteria and algae contain cytochrome c6, which is not found in higher plants and which may enhance survival in their planktonic mode of life. Cyanobacteria and algae contain another cytochrome, low-potential c549, which is not found in higher plants. This cytochrome has a structural role in PSII and may contribute to anaerobic survival. There is a third unique cytochrome, cytochrome M, in the planktonic photosynthesizers, and its function is unknown. New evidence is appearing to indicate evolution of cytochrome interaction mechanisms during the evolution of photosynthesis. The ease of cytochrome gene manipulation in cyanobacteria and in Chlamydomonas reinhardtii now provides great advantages in understanding of photosynthesis. The solution of tertiary and quaternary structures of cytochromes and cytochrome complexes will provide structural and functional detail at atomic resolution.

Photoreactions of cytochromes in algal chloroplasts

Light-induced cytochrome redox reactions were investigated with algal chloroplasts capable of high rates of electron transport coupled to phosphorylation. The electron-donor pool preceding photosystem I consists of membrane-bound cytochrome f-554.5 and a soluble cytochrome c-553; the latter replaces the function of plastocyanin of higher-plant chloroplasts. Both cytochromes are reduced by photosystem II and oxidized by photosystem I. A site of energy conservation precedes these c-type cytochromes. The data obtained with respect to the function of b-type cytochromes are comparable to those obtained with higher-plant chloroplasts. Cyclic electron transport is mediated by cytochrome b-563 in a photosystem-I-dependent reaction. In addition, cytochrome b-563 may be reduced by photosystem II, in accordance with recent findings with intact spinach chloroplasts. It therefore appears that cytochrome b-563 is a member of both cyclic and non-cyclic electron transport. In contrast to higher-plant chloroplasts, redox reactions of cytochrome b-559 are observable without any pretreatments. Cytochrome b-559, high-potential, is reduced by photosystem II through plastoquinone. In the presence of carbonyl cyanide m-chlorophenylhydrazone a cytochrome-b-559 oxidation by photosystem II is measured.

Isolation and characterization of soluble cytochrome c-553 and membrane-bound cytochrome f-553 from thylakoids of the green alga Scenedesmus acutus

Soluble cytochrome c-553 and membrane-bound cytochrome f-553 from the alga Scenedesmus acutus were purified to apparent homogeneity. The properties of cytochrome c-553 are comparable to preparations obtained from other eukaryotic algae, whereas the thylakoid-bound species resembles higher plant cytochrome f. Common characteristics are: 1. An asymmetrical alpha-band at 553 nm. 2. A midpoint redox potential of +38 MV (pH 7.0), with a pH dependency above pH 8.0 of -60mV/pH unit. 3. Formation of a pyridine hemochromogen with a maximum at 550 nm; no adducts with CN- or CO are observed. Distinguishing features are: 1. Cytochrome f-553 has a more complicated beta-band, with maxima at 531.5 and 524 nm, and hence a more complex low-temperature spectrum. Also the positions of the gamma- and delta-bank at 421.5 and 331 nm, respectively, distinguish cytochrome f-553 from cytochrome c-553, with gamma- and delta-bands at 416 and 318 nm. 2. The ferricytochrome c-553 spectrum exhibits a weak band at 692 nm, which is not observed with cytochrome f.

The role of plastidic cytochrome c in algal electron transport and photophosphorylation

By an improved isolation procedure chloroplasts could be obtained from the alga Bumilleriopsis filiformis (Xanthophyceae) which exhibited high electron transport rates tightly coupled to ATP formation. Uncouplers both stimulate electron transport and inhibit photophosphorylation. These chloroplasts retain almost all soluble cytochrome c-553 besides a membrane-bound cytochrome c-554.5 (=f-554.5). Sonification or iron deficiency removed the soluble cytochrome only with a concurrent decrease of electron transport from water to methyl viologen or to NADP and decreased non-cyclic and cyclic photophosphorylation. However, photosynthetic control and the P/2e ratios remain unaltered. In Bumilleriopsis, which apparently has no plastocyanin, the soluble cytochrome c-553 seemingly links electron transport between the bound cytochrome c and P-700.

The roles of c-type cytochromes in algal photosynthesis. Extraction from algae of a cytochrome similar to higher plant cytochrome f

A membrane-bound cytochrome resembling higher plant cytochrome f in many respects has been extracted from the algae Chlamydomonas. Euglena and Anacystis, and partially purified. The spectra of the cytochromes from Chlamydomonas and Euglena are virtually identical to that of parsley cytochrome f, with alpha-band maxima near 554 nm, very asymmetrical beta-bands, and gamma-band maxima at 421 nm. The cytochrome from Anacystis had alpha and gamma-bands both shifted to slightly longer wavelengths. The redox potential of the cytochrome from Chlamydomonas was determined as +350 mV, and its minimum molecular weight in sodium dodecyl sulphate as 31 000. The cytochrome from Euglena showed a rate of reaction with higher plant plastocyanin at least 100 times that of the soluble Euglena cytochrome c-552, and was unaffected by Euglena cytochrome c-552 antiserum. A very fast rate of electron transfer occurred between this cytochrome purified from Euglena and cytochrome c-552. The roles of the membrane-bound and soluble c-type cytochromes in algal photosynthesis are discussed, and it is recommended that the name cytochrome f should be reserved for the membrane-bound cytochrome (to emphasize its affinity with higher plant cytochrome f), while the soluble one should be named by its alpha-band (c-552, c-553, etc.) to make clear its distinctness from higher plant cytochrome f and homology with mitochondrial cytochrome c.

Reactions of plastocyanin and cytochrome 553 with photosystem I of Scenedesmus

Chloroplast material active in photosynthetic electron transport has been isolated from Scenedesmus acutus (strain 270/3a). During homogenization, part of cytochrome 553 was solubilized, and part of it remained firmly bound to the membrane. A direct correlation between membrane cytochrome 553 and electron transport rates could not be found. Sonification removes plastocyanin, but leaves bound cytochrome 553 in the membrane. Photooxidation of the latter is dependent on added plastocyanin. In contrast to higher plant chloroplasts, added soluble cytochrome 553 was photooxidized by 707 nm light without plastocyanin present. Reduced plastocyanin or cytochrome 553 stimulated electron transport by Photosystem I when supplied together or separately. These reactions and cytochrome 553 photooxidation were not sensitive to preincubation of chloroplasts with KCN, indicating that both redox proteins can donate their electrons directly to the Photosystem I reaction center. Scenedesmus cytochrome 553 was about as active as plastocyanin from the same alga, whereas the corresponding protein from the alga Bumilleriopsis was without effect on electron transport rates. It is suggested that besides the reaction sequence cytochrome 553 leads to plastocyanin leads to Photosystem I reaction center, a second pathway cytochrome 553 leads to Photosystem I reaction center may operate additionally.

Role of cyclic electron transport in photosynthesis as measured by the photoinduced turnover of P700 in vivo

The light-induced turnover of P700 was measured spectrophotometrically in a wide variety of algae and some photosynthetic mutants. Analysis of the postillumination recovery of P700+ revealed that the apparent first-order rate constant for reduction via the cyclic pathway was much lower that that via the noncyclic pathway. After activation of photosystems 1 and 2 the half-time for reduction of P700+ was 5-20 ms, whereas after activation of primarily photosystem 1 a longer half-time of ca. 150 ms was observed. The extent of the photooxidation of P700 was the same in both regimes of illumination. The longer half-time was also noted after inhibition of photosystem 2 by 3-(3,4-dichlorophenyl)-1,1-dimethylurea or mild heat shock and in mutant algae known to lack a functional photosystem 2. No signal was observed in mutants lacking P700 itself but those strains lacking either plastocyanin or cytochrome f were capable of a very slow turnover (reduction t 1/2 greater than 500 ms at room temperature). This very slow turnover was not affected by carbonyl cyanide m-chlorophenylhydrazone or the plastoquinone antagonist, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, indicating that the pathway for reduction of P700+ in these mutants is not energy linked and does not utilize the intersystem electron transport chain. The slow, 150 ms, reduction of P700+ due to cyclic flow was not observed when cells were engaged in photosynthesis at high-light intensities. The data are interpreted as evidence for the involvement of the total functional pool of P700 in both electron transport pathways, and we suggest that cyclic electron transport does not contribute to photosynthesis in oxygen-evolving autotrophs.

Algal glyceraldehyde-3-phosphate dehydrogenase. Pyridine-nucleotide requirements of two enzymes purified from Scenedesmus obliquus

Two enzymes with glyceraldehyde-3-phosphate dehydrogenase activity have been purified from heterotrophically grown Scenedesmus obliquus by ion-exchange chromatography and gel filtration. The D-enzyme has a molecular weight of 550000 and a VNADH: VNADPH ratio of 16 whereas the T-enzyme has a molecular weight of 140000 and a VNADH:VNADPH ratio of 0.15. The two enzymes, however, are very similar with regard to their Michaelis constants for the reduced pyridine nucleotides, pH optimum, subunit size and ultraviolet absorption.

A mutant strain of Scenedesmus obliquus deficient in ribulose diphosphate carboxylase, cytochrome f and photosystem II activity

The partial reactions of photosynthesis shown by strain F208, a non-photosynthetic mutant strain of Scenedesmus obliquus, have been compared with those performed by other mutant strains which lacked; Photosystem II activity (strains 11 and F131), cytochrome f (strain 50), P-700 and cytochrome f (strain F 119), and P-700 (strains F139 and 199). In this respect the properties of strain F208 were those that would be expected if Photosystem II activity and cytochrome f were not present in this strain. Examination of the composition of strain F208 has shown the absence of cytochrome f in both the soluble and the membrane-bound form. The considerably lower level of plastoquinone compared to that found in the wild type is characteristic of the strains which lack Photosystem II activities. Fraction 1 protein could not be detected in extracts of strain F208 by sedimentation velocity experiments in the ultracentrifuge, and only 7% of the wild type ribulose diphosphate carboxylase activity was found after chromatography of these extracts on DEAE-cellulose. The properties of strain F208 are compared with those of the ac-20 and cr-1 strains of Chlamydomanas rheinhardi, both of which have a deficiency of ribulose diphosphate carboxylase which is considered to result from a deficiency of chloroplast ribosomes. Strain F208 resembles these strains in its abnormal chloroplast ultrastructure and its decreased levels of the RNA forms derived from the chloroplast ribosomes when compared with the wild type. Chloroplast fragments isolated from strains of S. obliquus which lacked cytochrome f (strains 50 and F208) were able to use diaminodurene and ascorbate as an electron donor to Photosynstem I. Since this reaction was inhibited by mercuric salts it would appear that plastocyanin, but not cytochrome f, was involved in this electron transfer.

The kinetics and specificity of electron transfer from cytochromes and copper proteins to P700

The rates of electron transfer to P700 from plastocyanin and cytochrome f have been compared with those from three other c-type cytochromes and azurin, a copper protein resembling plastocyanin. Three different disruptive techniques were used to expose P700; digitonin, Triton X-100 and sonication. The following rate constants were measured at 25 degrees C, pH 7.0, with digitonin-treated chloroplasts: plastocyanin, 8 x 10(7)M(-1) x s(-1); red-algal cytochrome c-553, 1.9 x 10(7)M(-1) x s (-1); Pseudomonas cytochrome c-551, 8 x 10(6)M(-1) x s (-1); azurin, less than or = 3 x 10(5)M(-1) x s (-1); cytochrome f, less than or = 2 x 10(4)M(-1) x s (-1); mammalian cytochrome c, less than or = 2 x 10(4)M(-1) x s (-1). For electron transfer from plastocyanin, the effects of ionic strength, pH and temperature were also studied, and saturation effects found in earlier work were avoided by a full consideration of the various secondary reactions and inclusion of superoxide dismutase. The relative rates are discussed in relation to photosynthetic electron transport.

Temperature-sensitive mutations of the photosynthetic apparatus of Rhodospirillum rubrum

Temperature-sensitive mutants of Rhodospirillum rubum have been isolated by enrichment techniques selecting for conditionally aberrant electron flow in various portions of the electron transport scheme. The temperature sensitivity of a class of these strains is shown to preferentially affect the photosynthetic mode of growth and energy production over the aerobic mode.

Photoproduction of hydrogen by photosystem I of Scenedesmus

Anaerobically adapted and illuminated Scenedesmus evolves molecular hydrogen from endogenous organic compounds. This photoproduction of H2 does not require photosystem II, since 5x10(-6) M DCMU, which inhibited normal photosynthesis almost completely, did not significantly inhibit the photoevolution of H2. The relative efficiencies in far-red light of photosynthesis, photoreduction and H2 production were determined. Photohydrogen evolution was comparatively the most efficient of these three processes. Three mutants of Scenedesmus (isolated and characterized by Dr. N. I. Bishop) were also tested. Mutant PS-50, which lacks cytochrome 552, did not photoproduce H2. Mutant No. 11, blocked in photosystem II, showed rates of H2 production comparable to those of the wild type. Cl-CCP, an uncoupler of photophosphorylation, caused an apparent stimulation of H2 production by mutant No. 11 and wild-type cells. Mutant No. 8, which is partially blocked in photosystem I, showed a diminished photohydrogen production which was inhibited by Cl-CCP. These results suggest that photoproduction of hydrogen by photosystem I is due either to cyclic photophosphorylation, which supplies energy needed for a dark, H2-yielding reaction, or to a more direct photooxidation of organic compounds by the photosynthetic electron transfer chain.

Purification and some properties of a non-haem iron protein from the bacteroids of soya-bean (Glycine max Merr) nodules

A non-haem iron protein was isolated from an extract of soya-bean nodule bacteroids by a procedure including protamine sulphate and heat precipitation followed by chromatography on DEAE-cellulose. The purified protein contains non-haem iron and acid-labile sulphur and exhibits a spectrum with a rather broad absorption shoulder in the region 380-440nm and a more prominent peak at 280nm. From sedimentation-velocity measurements an apparent s(20,w) value of 1.3S was calculated. The protein functions as an electron carrier between the reducing system of illuminated chloroplast fragments and nitrogenase from nodule bacteroids, but it failed to function as a cofactor for the photochemical reduction of NADP in the presence of spinach chloroplasts. Also, it is inactive as a cofactor in the enzymic degradation of pyruvate to acetyl phosphate and CO(2) in the presence of a ferredoxin-free extract of Clostridium pasteurianum. Repeated freezing, storage and thawing of the non-haem iron protein resulted in a marked loss of activity in the photochemical acetylene-reduction assay. A major portion of the activity that was lost was restored as a result of treatment with sodium sulphide, mercaptoethanol and ferrous ammonium sulphate.