Recent studies on the chloroplast cytochrome b-559

An open reading frame encoding a putative haem-binding polypeptide is cotranscribed with the pea chloroplast gene for apocytochrome f

The nucleotide sequence of a 1 kbp region of pea chloroplast DNA upstream from the gene petA encoding apocytochrome f has been determined. An open reading frame of 231 codons (ORF231) encoding a putative membrane-spanning polypeptide is separated by 205 bp from the coding region of petA. The open reading frame is homologous to open reading frames located in a similar position with respect to petA in chloroplast DNA from Marchantia polymorpha, tobacco, rice, wheat and Vicia faba. The sequence around a conserved histidine residue in a putative membrane-spanning region of the polypeptide resembles sequences present in cytochrome b from chromaffin granules and neutrophil membranes, suggesting that the open reading frame may encode a haem-binding polypeptide, possibly a b-type cytochrome. Northern hybridisation analysis indicates the presence in pea chloroplasts of a complex pattern of transcripts containing ORF231. Large transcripts of 5.5 kb, 4.3 kb, 3.4 kb and 2.7 kb encode both ORF231 and apocytochrome f, indicating that ORF231 and petA are co-transcribed.

Comparative thermoluminescence study of autotrophically and photoheterotrophically cultivated Chlamydobotrys stellata

Thermoluminescence (TL) from autotrophically and photoheterotrophically cultivated Chlamydobotrys stellata was measured. Strong TL was emitted at 30°C after acetatenutrition of the alga. DCMU enhanced this band, as also did ferricyanide. It also appeared after poisoning of the alga with NH2OH or ANT-2p. These observations suggest that an alternative donor to photosystem II and not the water-splitting system is responsible for the TL + 30 band.

Photoreduction of QA, QB, and cytochrome b-559 in an oxygen-evolving photosystem II preparation from the thermophilic cyanobacterium Synechococcus sp

Light-induced absorption changes in an oxygen-evolving photosystem II (PS II) preparation from the thermophilic cyanobacterium Synechococcus sp. were analyzed using continuous illumination which caused the reduction of both QA (first stable quinone electron acceptor) and QB (second quinone electron acceptor of photosystem II). In this photosystem II preparation in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) the amount of QA was estimated to be 1 per 42 chlorophylls. In the absence of DCMU, plastoquinone (1.68 per QA) was photoreduced to plastohydroquinone within a few seconds, indicating that QB is reduced and protonated during this period. An electrochromic band shift centered around 685 nm was observed with and without DCMU. The extent of this band shift caused by QB reduction per electron was about a third or half of that caused by QA reduction. A significant amount of cytochrome b-559 (0.86 per QA) was photoreduced. Only 60% of the photoreduction of cytochrome b-559 was inhibited by a DCMU concentration that inhibited electron transfer beyond QB, indicating that the site of the reduction of cytochrome b-559 is located before the QB site and possibly on the donor side of PS II.

The relationship of the cyclic and non-cyclic electron transport pathways in chloroplasts

Light-induced redox changes of plastocyanin, the Rieske iron-sulfur center, and P-700 have been studied in situ in spinach chloroplasts. Plastocyanin and the Rieske center behaved in an analogous manner in that their steady states were fully oxidized in the light in the presence or absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea when an electron acceptor is present. After illumination under conditions of non-cyclic electron transfer from water to an electron acceptor, followed by a short-dark period, the steady state of both shifted to a more reduced level. A 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive photo-reduction of the Rieske center was observed in ferricyanide-washed chloroplast fragments. With reduced ferredoxin as electron donor, it was possible to demonstrate a reduction in the dark of these electron carriers and of P-700; this reduction was insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea but was inhibited by antimycin A. These findings are discussed in relation to a function for these electron carriers in the cyclic electron transport pathway in chloroplasts and to their function in the non-cyclic electron transport pathway.

Interactions between photosystem II components in chloroplast membranes. A correlation between the existence of a low potential species of cytochrome b-559 and low chlorophyll fluorescence in inhibited and developing chloroplasts

1. Chloroplasts inhibited by incubation with hydroxylamine in the light exhibit a low fluorescence yield upon illumination in the presence of dithionite sufficient to completely reduce the primary acceptor, Q. In the absence of magnesium ions, the fluorescence yield is the same as in control chloroplasts, suggesting that the reason for the low yield is a defect in the mechanism by which Mg2+ enhances the fluorescence. These chloroplasts were previouly shown to contain only low potential (Em7.8 = +80 mV) cytochrome b-559 (Horton, P. and Croze, E (1977) Biochim. Biophys. Acta 462, 86-101). 2. In Photosystem II particles, in heat-treated chloroplasts and in trypsin-digested chloroplasts, high potential cytochrome b-559 is absent and the variable fluorescence yield is again low. 3. Peas grown under intermittent light contain only one-fifth of the content of high potential cytochrome b-559 seen in fully greened plants, yet show high rates of water to methyl viologen electron transport. Aquisition of the high potential cytochrome b-559 accompanies synthesis of chlorophyll b, the onset of Mg-stimulated fluorescence and an increased variable yield of fluorescence. A similar correlation was seen during greening of dark-grown barley. 4. It is proposed that the high potential state of cytochrome b-559 is due to the same membrane properties which allow cation enhanced variable fluorescence, so that the presence of low potential cytochrome b-559 is accompanied by a decrease in variable fluorescence yield.

Some photoreactions of isolated cytochrome b-559

Cytochrome b-559 was isolated from spinach and the alga Bumilleriopsis filiformis (Xanthophyceae) and characterized by functional properties: (a) It was active as electron acceptor in a diaphorase system using NADPH as donor and ferredoxin and ferredoxin-NADP reductase as redox proteins. (b) It exhibited photooxidation with Photosystem-I particles, when illuminated with 707 nm light. (c) It was photooxidized by Photosystem-II particles and 652 nm light at room temperature. Light greater than 702 nm was ineffective. The data corroborate previous reports on redox reactions of bound cytochrome b-559.

The relationship between the activity of chloroplast photosystem II and the midpoint oxidation-reduction potential of cytochrome b-559

The role of cytochrome b-559 in Photosystem II reactions has been investigated using hydroxylamine treatment of chloroplast membranes. Incubation of chloroplasts with hydroxylamine in darkness resulted in inhibition of water oxidation and a decrease in the amplitude of cytochrome b-559 reducible by hydroquinone. The loss of water oxidizing activity perfectly correlated with the decrease in amplitude of cytochrome b-559 reduction. Potentiometric titration of cytochrome b-559 after hydroxylamine treatment revealed a component with Em7.8 at +240 mV in addition to a lower potential species at +90 mV. This compared to control chloroplasts in which cytochrome b-559 exists in the typical high potential state, Em7.8 = +383 mV, in addition to some of the low potential (Em7.8 = +77 mV) form. Photosystem II activity could be further inhibited by incubation with hydroxylamine in the light. In these chloroplasts only low rates of photooxidation of artificial electron donors were observed compared to 'dark' chloroplasts. In addition, the hydroxylamine light treatment caused a further change in cytochrome b-559 redox properties; a single component, Em7.8 = 90 mV is seen in titration curves. The role of cytochrome b-559 in Photosystem II functioning is discussed on the basis of these observations which suggest a dependence of photooxidizing ability of Photosystem II on the redox properties of this cytochrome.

Further evidence for the electron pool hypothesis. The effect of KCN and DSPD on the photophobic reaction in the filamentous blue-green alga Phormidium uncinatum

In continuation of experiments with photo-system II inhibitors [3-(3,4-dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropylbenzo-quinone] the effect of photosystem I inhibitors was studied. 1. Neither the plastocyanin inhibitor, potassium cyanide, nor the ferredoxin antagonist, disalicyliden propandiamin, markedly affected those phobic reactions which are mediated by the electron transport via photosystem II into the electron pool. 2. On the other hand those phobic reactions, which are triggered by an increased flow of electrons out of the pool, are specifically inhibited by both substances. These results are regarded as further evidence that there is only one electron pool, the level of which triggers photophobic reactions and is located in the linear electron transport chain near photosystem II.

Stimulation of photosystem I-induced oxidation of chloroplast cytochrome b-559 by pre-illumination and by low pH

(1) The proportion of higher plant chloroplast cytochrome b-559 oxidizable during illumination by low intensity 732 nm light increases as the pH is decreased below 6.5. At pH 5.0-5.3 total oxidation is seen and subsequent red light can cause reduction of up to 2/3 of the oxidized cytochrome. The oxidation by far red light at pH 5 is inhibited by 2 muM 2,5-dibromo-3-methyl-6-isopropyl-rho-benzoquinone whereas the red light-induced reduction is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In this pH range ferricyanide-oxidized cytochrome b-559 exists in a form not reducible by ferrocyanide. (2) An increase in the amplitude of far-red induced oxidation also occurs at higher pH (up to pH 7.8) after pre-treatment of chloroplasts with substantially higher levels of light (approx. 10(6) ergs-cm-2-s-1). The degree of light activation is pH dependent, being more pronounced at lower pH. After light activation, cytochrome b-559 can be completely oxidized by far-red light in a manner reversible by red light up to pH values of 6, and the curve describing the amplitude of far-red oxidation as a function of pH is shifted by 0.5-1.0 pH unit toward higher pH. Far-red oxidation and red light reduction are again inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, respectively. (3) Light activation at pH 5.2-6.0 is also manifested in a small decrease in the amplitude of subsequent dark ferrocyanide reduction, and this decrease is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (10 muM). (4) The effect of intramembranal acidity on the effective redox potential of cytochrome b-559 and its function is discussed.