Light-induced redox changes of cytochrome b-559

Regulation of protein metabolism: Coupling of photosynthetic electron transport to in vivo degradation of the rapidly metabolized 32-kilodalton protein of the chloroplast membranes

In Spirodela oligorrhiza, mature chloroplasts copiously synthesize and degrade a 32-kilodalton membrane protein. The rates of synthesis and degradation are controlled by light intensity, the protein being unstable in the light and stable in the dark. Light-driven synthesis, but not degradation, is dependent on ATP. Degradation is blocked by herbicides inhibiting photosystem II electron transport, such as diuron and atrazine. Thus, both anabolism and catabolism of the 32-kilodalton protein are photoregulated, with degradation coupled to electron transport rather than phosphorylation.

Evidence for a biological role in photosynthesis for cytochrome b-559--a component of photosystem II reaction center

Absorbance changes in untreated intact leaf discs, produced upon excitation with high-intensity red light, were shown to be due to the photooxidation of cytochrome b-559. At low intensities (<100 W/m2), photooxidation was almost undetectable. Photooxidation occurred with a half-time of 4.3 sec and an extent of 0.64 mol of cytochrome per 320 mol of chlorophyll. Upon transition to darkness, an additional oxidation occurred that exhibited faster kinetics (t/12 < 100 msec) and 0.32 mol of cytochrome was oxidized per 320 mol of chlorophyll. Photooxidation was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea and was specifically induced by red light since far-red light did not cause any absorbance decrease. These results suggest that the redox changes of cytochrome b-559 are driven by photosystem II. Photooxidation was increased by 67% and its initial rate was doubled upon incubation of the leaf in carbonylcyanide p-trifluoromethoxy-phenylhydrazone. Exposure of the leaf to mild water stress or mild heat stress resulted in an increase in the extent of photooxidation and in a 6-fold decrease in the rate constant. Mild heat stress also induced a large increase of the rate constant for the dark reduction of the cytochrome. The dependence of photooxidation on high-intensity red light, its inhibition criteria, the fast transient dark oxidation, and enhancement of both photooxidation and dark transient oxidation by treatments that affect Z, the primary donor to P680, indicate that cytochrome b-559 in vivo is involved in cyclic electron flow around photosystem II. Its primary role in photosynthesis is to divert excess photons from a linear to a cyclic electron flow at high light intensities for protection of the D1 and D2 proteins against photodamage. Dark oxidation of the cytochrome is suggested to reflect a second role, that of deactivation of the powerful oxidant Z+ in the dark.

Post-illumination kinetics of cytochrome f reduction in chloroplast thylakoid in the presence of dibromothymoquinone

Addition of dibromothymoquinone (DBMIB) to isolated chloroplast thylakoids reduces cytochrome f in the dark. Reduced cytochrome f is oxidised when the thylakoids are illuminated, and is re-reduced in the subsequent darkness. The rate of re-reduction in the dark is faster after red (650 nm) illumination than after far red (713 nm) illumination. In the presence of DCMU or upon heat treatment or at high (greater than 10 microM) concentration of DBMIB the rate of dark reduction after red illumination becomes slower and equal to that after far red illumination, suggesting that photosystem II electron transfer at least upto plastoquinone facilitates DBMIB-mediated reduction of cytochrome f in the thylakoids.

Photooxidation of the cytochrome b-559 in the presence of various substituted 2-anilinothiophenes and of some other compounds, in Chlamydomonas reinhardtii

Five substituted 2-anilinothiophenes and two substituted carbonylcyanide-phenylhydrazones were comparatively studied with respect to their capacities for inducing photooxidation of the cytochrome b-559 in chloroplast fragments and in whole cells of Chlamydomonas reinhardtii (wild type and P-700-lacking mutant Fl 5). In addition, some other compounds: antimycin A, picric acid, tetraphenylboron and NH4Cl were also tested. Cytochrome b-559 photooxidations were clearly observed in the presence of 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene (ANT 2p), 2-(3,4,5-trichloro)anilino-3,5-dinitrothiophene (ANT 2s), 2-(4-chloro)anilino-3,5-dinitrothiophene and, with greater amplitudes, in the presence of carbonylcyanide-p-trifluoromethoxyphenylhydrazone and carbonylcyanide-m-chlorophenylhydrazone, both in whole cells and in chloroplast fragments. Picric acid, antimycin A and tetraphenylboron were also able to induce cytochrome b-559 photooxidation in chloroplast fragments, but not in whole cells. In the wild type, the highest photoinduced redox changes were 1.1 (carbonylcyanide-p-trifluoromethoxyphenylhydrazone, carbonylcyanide-m-chlorophenyl-hydrazone) and 0.6 (ANT 2p, ANT 2s) mumol of oxidized cytochrome b-559/1 mmol of chlorophyll, corresponding to 40% and 23% of the redox changes which could be induced chemically. All these cytochrome b-559 photooxidations, the greater part of which was inhibited by 3-(3,4-dichloropheny)-1,1-dimethylurea and occurred in the mutant Fl 5, appeared to be mainly Photosystem II-dependent reactions. But 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive Photosystem I-dependent photooxidations of cytochrome b-559 occurred also in the wild type. On the other hand, 2-(4-dimethylamine)-anilino-3,5-dinitrothiophene, 2-N-methyl-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene and NH4Cl did not induce any cytochrome b-559 photooxidation. These results were discussed taking in consideration the nature of the molecular substitutions of the various tested substances and their respective acceleration of the deactivation reactions of the water-splitting enzyme system Y of photosynthesis capacities which had been defined elsewhere by Renger (Renger, G. (1972) Biochim. Biophys. Acta 256, 428-439) for spinach chloroplasts. Like the acceleration of the deactivation reactions of the water-splitting enzyme system Y effect, the capacity for inducing the cytochrome b-559 photooxidation appeared dependent on the acidity of the NH group and on the number of halogenous substituents in the aromatic ring of the molecule. The greatest action towards cytochrome b-559 photooxidation was obtained with the most active acceleration of the deactivation reactions of the water-splitting enzyme system Y agents: carbonylcyanide-p-trifluoromethoxyphenylhydrazone, ANT 2p and ANT 2s.

Photoreactions of Cytochrome b-559 and cyclic electron flow in photosystem II of intact chloroplasts

The high potential cytochrome b-559 of intact spinach chloroplasts was photooxidized by red light with a high quantum efficiency and by far-red light with a very low quantum efficiency, when electron flow from water to Photosystem II was inhibited by a carbonyl cyanide phenylhydrazone (FCCP or CCP). Dithiothreitol, which reacts with FCCP or CCCP, reversed the photooxidation of cytochrome b-559 and restored the capability of the chloroplasts to photoreduce CO2 showing that the FCCP/CCCP effects were reversible. The quantum efficiency of cytochrome b-559 photooxidation by red or far-red light in the presence of FCCP was increased by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone which blocks oxidation of reduced plastoquinone by Photosystem I. When the inhibition of water oxidation by FCCP or CCP was decreased by increased light intensities, previously photooxidized cytochrome b-559 was reduced. Red light was much more effective in photoreducing oxidized high potential cytochrome b-559 than far-red light. The red/far-red antagonism in the redox state of cytochrome b-559 is a consequence of the different sensitivity of the cytochrome to red and far-red light and does not indicate that the cytochrome is in the main path of electrons from water to NADP. Rather, cytochrome b-559 acts as a carrier of electrons in a cyclic path around Photosystem II. The redox state of the cytochrome was shifted to the oxidized side when electron transport from water became rate-limiting, while oxidation of water and reduction of plastoquinone resulted in its shifting to the reduced side.

[Cytochrome b-559 photooxidation in the presence of carbonyl cyanide p-trifluorometh-oxyphenylhydrazone and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone or p-benzoquinone in three non-photosynthetic mutants of Chlamydomonas reinhardti (author's transl)]

Studies of absorbance related to the cytochrome b-559 photooxidation induced by FCCP, with and without addition of 3-p-chlorophenyl-1, 1-dimethylurea (CMU), DBMIB or p-benzoquinone, in whole cells and in chloroplast fragments of Chlamydomonas reinhardti, were carried out. In addition to the wild type, three strains of non-photosynthetic mutants were used: Fl 5, which lacks P 700; Fl 9 and Fl 15, which are deficient in bound cytochrome c-553 and in cytochrome b-563. In the presence of FCCP, whole cells and chloroplast fragments of the four strains showed a System II-dependent photooxidation of cytochrome b-559. This photooxidation was inhibited by CMU but it occurred again in presence of FCCP, CMU and DBMIB. In chloroplast fragments, cytochrome b-559 photooxidation was also inhibited by an excess of FCCP; it was recovered, likewise, by addition of DBMIB. In whole cells, the highest measured redox changes were: 1 mu mol oxidized cytochrome b-559 per 1 mmol chlorophyll, corresponding approximately to about one seventh (wild type, Fl5) or one fifth (Fl 9, Fl 15) of the total amount of this cytochrome. Another kind of cytochrome b-559 photooxidation, CMU-insensitive, also occurred in the mutants Fl 9 and Fl 15 and in the wild type, but not in the mutant Fl 5. This latter kind of photooxidation was observed with chloroplast fragments in the presence of FCCP and CMU and also with whole cells in the presence of FCCP, CMU and p-benzoquinone. These reactions can be attributed to the Photosystem I; they do not require the intervention of the cytochrome c-553. A high-potential form of cytochrome b-559, hydroquinone-reducible, was involved in these two kinds of photooxidation. In addition, a lower potential form, reducible only by ascorbate, appeared to be able to interfere also. An interpretation is attempted, taking into consideration the various effects of FCCP and DBMIB, at different concentrations, on photosynthetic electron transport.