Physiological factors determining formation of plastocyanin and plastidic cytochrome c-553 in Scenedesmus

Inactivation of iron-sulfur cluster biogenesis regulator SufR in Synechocystis sp. PCC 6803 induces unique iron-dependent protein-level responses

BACKGROUND

Iron-sulfur (Fe-S) clusters are protein-bound cofactors associated with cellular electron transport and redox sensing, with multiple specific functions in oxygen-evolving photosynthetic cyanobacteria. The aim here was to elucidate protein-level effects of the transcriptional repressor SufR involved in the regulation of Fe-S cluster biogenesis in the cyanobacterium Synechocystis sp. PCC 6803.

METHODS

The approach was to quantitate 94 pre-selected target proteins associated with various metabolic functions using SRM in Synechocystis. The evaluation was conducted in response to sufR deletion under different iron conditions, and complemented with EPR analysis on the functionality of the photosystems I and II as well as with RT-qPCR to verify the effects of SufR also on transcript level.

RESULTS

The results on both protein and transcript levels show that SufR acts not only as a repressor of the suf operon when iron is available but also has other direct and indirect functions in the cell, including maintenance of the expression of pyruvate:ferredoxin oxidoreductase NifJ and other Fe-S cluster proteins under iron sufficient conditions. Furthermore, the results imply that in the absence of iron the suf operon is repressed by some additional regulatory mechanism independent of SufR.

CONCLUSIONS

The study demonstrates that Fe-S cluster metabolism in Synechocystis is stringently regulated, and has complex interactions with multiple primary functions in the cell, including photosynthesis and central carbon metabolism.

GENERAL SIGNIFICANCE

The study provides new insight into the regulation of Fe-S cluster biogenesis via suf operon, and the associated wide-ranging protein-level changes in photosynthetic cyanobacteria.

Plastocyanin/cytochrome c6 interchange in Scenedesmus vacuolatus

Plastocyanin and cytochrome c6 from the green alga Scenedesmus vacuolatus were immunoquantified in cells grown under different concentrations of copper and iron. Plastocyanin expression was constitutive, its synthesis was not significantly affected by iron availability, and increases with copper availability. On the contrary, cytochrome c6 synthesis is repressed by copper, and only residual amounts of the protein were detected at 0.1 micromol/L copper. Under copper deficiency, cytochrome c6 is slightly dependent on iron. In natural environments, plastocyanin seems to be the predominant electron donor to P700.

Electrochromic absorbance changes in the chlorophyll-c-containing alga Pleurochloris meiringensis (Xanthophyceae)

Flash-induced absorbance changes were measured in the Chl-c-containing alga Pleurochloris meiringensis (Xanthophyceae) between 430 and 570 nm. In addition to the bands originating from redox changes of cytochromes, three major positive and tow negative transient bands were observed both 0.7 and 20 ms after the exciting flash. These transient bands peaking at 520, 480 and 451 nm and 497 and 465 nm, respectively, could be assigned to an almost homogeneous shift of the absorbance bands with maxima at 506, 473 and 444 nm, respectively. The shape of the absorbance transients elicited from PS I or PS II was identical, and the two photosystems contributed nearly equally to the absorbance changes. Furthermore, the decay transients were sensitive to the preillumination of the cells. These data strongly suggest that the absorbance transients originate from an electrochromic response of carotenoid molecules. The pigment species responsible for the 506 nm absorption band, probably heteroxanthin or diatoxanthin, transferred excitation energy to both photosystems as shown by the aid of 77 K fluorescence excitation spectra.

Characterization of a folding intermediate of apoplastocyanin trapped by proline isomerization

The unfolding and refolding transitions of French bean apoplastocyanin (apo-Pc), a beta-sandwich protein, have been characterized. The apoprotein is stabilized by sodium sulfate and can be reversibly unfolded by guanidine hydrochloride (GuHCl). However, in contrast to holo-Pc, apo-Pc is unstable at low ionic strength, suggesting that the copper ion stabilizes the holoprotein. The equilibrium unfolding transition monitored by peptide circular dichroism (CD) and tyrosine fluorescence is described by a two-state model. The kinetics of the unfolding transition were monitored using a manual mixing technique and are consistent with a single two-state transition. In contrast, the kinetics of the refolding reaction measured by fluorescence and CD show two transitions with different rates. The relaxation time of the slower phase (800-1000 s) is almost independent of GuHCl concentration. The faster phase was observed only under strongly native conditions, and its relaxation time is GuHCl-dependent. Double-jump experiments and acceleration by cyclophilin demonstrate that both phases involve cis-trans isomerization of proline residues. The changes in fluorescence associated with the two phases are more than 150% of the total change expected from equilibrium experiments, indicating the presence of intermediate(s) with fluorescence greater than the unfolded state. Amide hydrogen-exchange experiments coupled with two-dimensional NMR spectroscopy demonstrate the formation of an intermediate in the very low refolding reaction in which amide protons in the beta-sheets are weakly protected from exchange. No CD evidence for nativelike beta-sheet formation was found for this intermediate. The NMR experiments suggest that the intermediate is compact with flexible beta-sheets and altered packing of the hydrophobic core. It has many of the characteristics of a molten globule. However, the 1H NMR spectrum of the intermediate exhibits a small number of shifted resonances that indicate the presence of specific tertiary interactions in a localized region. A mechanism for refolding of apoplastocyanin is proposed that includes two slow steps corresponding to trans-->cis isomerization of two prolines.

Steady-state kinetics of the photosystem I reaction in chloroplasts of Dunaliella which contain variable concentrations of plastocyanin

The endogenous plastocyanin (PC) concentrations of Dunaliella cultures were varied from 0.3 to 3.1 molecules per pigment 700 (P700) by decreasing the Cu(+) supply of the nutrient. With these cultures the amount of PC which is sufficient for maximum photosynthesis in intact cells was determined to be about 1 to 1.5 PC/P700. Chloroplasts were also prepared from these cells and were employed in enzyme kinetic measurements of the PSI reaction from ascorbate reduced diaminodurene (DAD) to methylviologen/O2. The k m value for DAD in this reaction was 106 μM. A decrease of the endogenous PC concentration caused no change of the k m value but affected the V max in the DAD-dependent reaction. A similar interference of the PC concentration on the maximum reaction rate could also be observed when the light intensity was varied.

Isolation of chlorophyll-protein complexes and quantification of electron transport components in Synura petersenii and Tribonema aequale

The chlorophyll-protein complexes of the yellow alga Synura petersenii (Chrysophyceae) and the yellow-green alga Tribonema aequale (Xanthophyceae) were studied. The sodiumdodecylsulfate/sodiumdesoxycholate solubilized photosynthetic membranes of these species yielded three distinct pigment-protein complexes and a non-proteinuous zone of free pigments, when subjected to SDS polyacrylamid gel electrophoresis. The slowest migrating protein was identical to complex I (CP I), the P-700 chlorophyll a-protein, which possessed 60 chlorophyll a molecules per reaction center in Tribonema and 108 in Synura. The zone of intermediate mobility contained chlorophyll a and carotenoids. The absorption spectrum of this complex was very similar to the chlorophyll a-protein of photosystem II (CP a), which is known from green plants. The fastest migrating pigment protein zone was identified as a light-harvesting chlorophyll-protein complex. In Synura this protein was characterized by the content of chlorophyll c and of fucoxanthin. Therefore this complex will be named as LH Chl a/c-fucocanthin protein. In addition to the separation of the chlorophyll-protein complexes the cellular contents of P-700, cytochrome f (bound cytochrome) and cytochrome c-553 (soluble cytochrome) were measured. The stoichiometry of cytochrome f: cytochrome c-553:P-700 was found to be 1:4:2.4 in Tribonema and 1:6:3.4 in Synurá.

Isolation of chlorophyll-protein complexes and quantification of electron transport components in Synura petersenil and Tribonema aequale

The chlorophyll-protein complexes of the yellow alga Synura petersenii (Chrysophyceae) and the yellow-green alga Tribonema aequale (Xanthophyceae) were studied. The sodiumdodecylsulfate/sodiumdesoxycholate solubilized photosynthetic membranes of these species yielded three distinct pigment-protein complexes and a non-proteinous zone of free pigments, when subjected to SDS polyacrylamid gel electrophoresis. The slowest migrating protein was identical to complex I (CP I), the P-700 chlorophyll a-protein, which possessed 60 chlorophyll a molecules per reaction center in Tribonema and 108 in Synura. The zone of intermediate mobility contained chlorophyll a and carotenoids. The absorption spectrum of this complex was very similar to the chlorophyll a-protein of photosystem II (CP a), which is known from green plants. The fastest migrating pigment protein zone was identified as a light-harvesting chlorophyll-protein complex. In Synura this protein was characterized by the content of chlorophyll c and of fucoxanthin. Therefore this complex will be named as LH Chl a/c-fucocanthin protein. In addition to the separation of the chlorophyll-protein complexes the cellular contents of P-700, cytochrome f (bound cytochrome) and cytochrome c-553 (soluble cytochrome) were measured. The stoichiometry of cytochrome f: cytochrome c-553:P-700 was found to be 1:4:2.4 in Tribonema and 1:6:3.4 in Synurá.

Influence of light on plastocyanin formation in the alga Scenedesmus acutus

Scenedesmus acutus, pregrown autotrophically, forms high levels of plastocyanin during an initiation period of 24 hr in the light, after cupric ions have been added to depleted cells. In the dark, no plastocyanin formation is observed. This is in contrast to heterotrophic cells which, over the same period and under identical conditions, yield about half the plastocyanin level regardless of whether they are incubated in the dark or in the light. The ATP level is high in all cases when substantial formation of plastocyanin takes place, while the (35)S-sulfolipid level, the marker of thylakoid formation, is at variance. After the 24-hr plastocyanin formation period, the (35)S-sulfolipid level is high in illuminated autotrophic cells as well as in dark-incubated heterotrophic ones, but low in illuminated heterotrophic cells, all of them having substantial plastocyanin contents. Additional experiments on light dependence of copper uptake, influence of light quality, occurrence of apoplastocyanins, provide a set of data to assume that the role of light in plastocyanin formation is primarily through photophosphorylation, the ATP presumably being necessary for processing of plastocyanin precursors to holoprotein.

Formation of silver plastocyanin in Scenedesmus

Silver ions up to 5 microM do not affect growth of the green microalga Scenedesmus acutus. They induce formation of protein species precipitable by an antibody specific against plastocyanin. The metal is incorporated into a part of the induced protein in competition with copper. Bismuth, lead and molybdenum had no effect. The amount of both silver- and copper-containing plastocyanins so formed apparently regulates concurrently inhibition of soluble plastidic cytochrome c-553. The silver-copper competition for the build-up of blue plastocyanin can be shown with intact cells, not with isolated algal plastocyanin.

Components and activity of the photosynthetic electron transport system of intact heterocysts isolated from the blue-green alga Nostoc muscorum

Heterocysts of the blue-green alga Nostoc muscorum have been isolated by prolonged treatment with lysozyme. Quantitative data are presented which show the occurrence of cytochromes c-553, f-557 and b-563 in heterocysts in amounts comparable to vegetative cells. Particularly the content of the water-soluble cytochrome c-553 can be used to evaluate the intactness of a heterocyst preparation. Cytochrome f-557 has been partially purified and found to be a c-type cytochrome corresponding to cytochrome f of higher plants and other algae. Cytochrome b-559 is present in vegetative cells but not in heterocysts. The content of plastoquinone in heterocysts is reduced to 42% of the amount present in vegetative cells. These data suggest a degradation of Photosystem II during heterocyst differentiation. Measurements of photosynthetic electron transport in heterocysts proved the inability of the photosynthetic apparatus to carry out electron transport with electrons donated by water or diphenylcarbazide. In Tris-washed thylakoids from vegetative cells, however, diphenylcarbazide can act as an electron donor to Photosystem II.

Reciprocal formation of plastocyanin and cytochrome c-553 and the influence of cupric ions on photosynthetic electron transport

The green alga Scenedesmus acutus is able to synthesize plastocyanin and cytochrome c-553. The concentrations of plastocyanin and cytochrome c-553 vary inversely in response to the cupric-ion concentrations of the growth medium (Bohner, H. and Böger, P. (1978) FEBS Lett. 85, 337-339). Both proteins form a homogeneous donor pool to the reaction center of Photosystem I. This donor pool can be varied quantitatively and qualitatively by different growth conditions. These variations have no influence on algal growth or photosynthetic electron transport as measured in vivo by oxygen evolution, fluorescence induction and cytochrome f-553 and c-553 redox reactions using Cu2+ concentrations of less than 10 microM in the culture medium. At higher cupric-ion concentrations, which already retard algal growth, specific sites of the photosynthetic electron-transport chain are affected: the oxidizing side of Photosystem II and the reducing side of Photosystem I.

High variability of the electron carrier plastocyanin in microalgae

1. The plastocyanin content of Scenedesmus can be dramatically varied with the copper content of the culture medium. Figures as high as 7 mmol plastocyanin/mol chlorophyll are possible. Electron paramagnetic resonance (EPR) spectroscopy has been used to determine this physiological response quantitatively in intact cells having different amounts of plastocyanin. The results obtained by the EPR technique were compared with data on isolated plastocyanin determined either by spectrophotometry or immunoelectrophoresis. Agreement was found for the amount of plastocyanin detected by the first two methods, whereas the last assay yielded data at least 25% higher on the average. Under all culture conditions a copper-free plastocyanin precursor is present. 2. The EPR properties of purified plastocyanin and those of cellular plastocyanin located within the thylakoids are practically identical in terms of g-values, hyperfine splittings, signal linewidths and saturation behavior at temperatures of 12--15 K. Our data indicate that plastocyanin is not present in a membrane-bound form but exists as a single soluble pool. 3. The studies have been extended to the algae Dunaliella parva and Bumilleriopsis filiformis. The first species exhibits a limited variation of plastocyanin with the copper content of the medium. Furthermore, no cytochrome c-553 could be detected in Dunaliella even under conditions of copper deficiency. In contrast, Bumilleriopsis does not contain plastocyanin regardless of the amount of copper offered.