Berah D. McSwain

Photochemical activity and components of membrane preparations from blue-green algae. I. Coexistence of two photosystems in relation to chlorophyll a and removal of phycocyanin.

Abstract Nostoc muscorum (Strain 7119) cells were disrupted and the accessory pigment phycocyanin was removed from membrane fragments by digitonin treatment. The phycocyanin-depleted membrane fragments retained both Photosystem I and Photosystem II activity, as evidenced by high rates of NADP+ photoreduction either by water or by reduced 2,6-dichlorophenolindophenol, indicating that phycocyanin is not an essential component for electron transport activity. No separation of the two photosystems was effected by the digitonin treatment. Even drastic digitonin treatments failed to diminish significantly the remarkably stable electron transport from water to NADP+. Action spectra and relative quantum efficiency measurements demonstrated the existence of both Photosystem I and Photosystem II in membrane fragments which contained chlorophyll a as the only significant light-absorbing pigment.

THREE LIGHT REACTIONS AND THE TWO PHOTOSYSTEMS OF PLANT PHOTOSYNTHESIS

Abstract— Recent work in our laboratory yielded new evidence that noncyclic electron transport in chloroplasts from water to ferredoxin (Fd) and N ADP is carried out solely by System II which, unexpectedly, was found to include not one but two photoreactions (IIa and IIb). The evidence suggests that these operate in series, being joined together by a ‘dark’ chain of electron carriers that includes (but is not limited to) cytochrome b559 and plastocyanin (PC):

Effects of magnesium and chloride ions on light-induced electron transport in membrane fragments from a blue-green alga☆

The effects of magnesium and chloride ions on photosynthetic electron transport were investigated in membrane fragments of a blue-green alga, Nostoc muscorum (Strain 7119), noted for their stability and high rates of electron transport from water or reduced dichlorophenolindophenol to NADP+. Magnesium ions were required not only for light-induced electron transport from water to NADP+ but also for protection in the dark of the integrity of the water-photooxidizing system (Photosystem II). Membrane fragments suspended in the dark in a medium lacking Mg2+ lost the capacity to photoreduce NADP+ with water on subsequent illumination. Chloride ions could substitute, but less effectively, for each of these two effects of Chloride ions could substitute, but less effectively, for each of these two effects of magnesium ions. By contrast, the photoreduction of NADP+ by DCIPH2 was independent of Mg2+ (or Cl-) for the protection of the electron transport system in the dark or during the light reaction proper. Furthermore, high concentration of MgGl2 produced a strong inhibition of NADP+ photoreduction with DCIPH2 without significantly affecting the rate of NADP+ photoreduction with water. The implications of these findings for the differential involvement of Photosystem I and Photosystem II in the photoreduction of NADP+ with different electron donors are discussed.

Evidence for two types of P-700 in membrane fragments from a blue-green alga☆

The mathematical analysis described in the preceding paper (Biochim. Biophys. Acta (1977) 460, 65-75), in which the steady-state photooxidation of P-700 was compared with overall electron flux in Photosystem I chloroplast fragments, was applied to membrane fragments from the blue-gree alga Nostoc muscorum (Strain 7119) noted for their high activity of both Photosystem I and Photosystem II. The same analysis, which gave good agreement between the photooxidation of P-700 and the overall light-induced electron flux (measured as NADP+ reduction) in Photosystem I chloroplast fragments, revealed in the algal membrane fragments two P-700 components: one responding to high light intensity (P-700 HI), the photooxidation of which was in good agreement with the overall electron flux (measured as NADP+ reduction by reduced 2,6-dichlorophenolindophenol), and the other component responding to low light intensity (P-700 LI), the photooxidation of which was not correlated with the reduction of NADP+ by reduced 2,6-dichlorophenolindophenol.

Action spectra of the three light reactions in photosynthesis.

Abstract Action spectra for the photoreduction of cytochrome b559 and C550 and for the photooxidation of cytochromes b559 and f in spinach chloroplasts were measured in the region from 640 to 720 nm. The action spectra for the photoreduction of cytochrome b559 and C550 are closely similar and show marked decreases in efficiency at wavelengths longer than 680 nm. These findings are consistent with the conclusion that the two components are reduced by the same photoact of Photosystem II. The action spectrum for the photooxidation of cytochrome b559 also showed a decrease in efficiency at wavelengths longer than 680 nm, characteristic of Photosystem II, but the photooxidation of cytochrome f showed an increase in efficiency at long wavelengths, characteristic of Photosystem I. These action spectra show a pattern consistent with a recently proposed concept of three light reactions in plant photosynthesis.

Factors influencing photosynthetic enhancement in isolated chloroplasts

Abstract Photosynthetic enhancement of oxygen evolution (linked to CO 2 assimilation) in isolated chloroplasts was found to be governed by the supply of ATP. The addition of ATP (but not AMP) abolished enhancement that consistently occurred without added ATP. Enhancement in the H 2 O → NADP reaction by chloroplasts was investigated in the light of one recent report that the phenomenon occurs when pure ferredoxin is replaced by a crude preparation (PPNR) and another report that the phenomenon is governed by Mg ++ concentration. Fractionation of PPNR led to the isolation of a protein factor which when added to pure ferredoxin induced enhancement. However, the rate of NADP reduction with pure ferredoxin and without enhancement was greater than the maximum rate of NADP reduction with enhancement induced by either the protein factor of PPNR. The report that Mg ++ concentration governs enhancement was not confirmed.

Effect of NADP+ on light-induced cytochrome changes in membrane fragments from a blue-green alga☆

The effect of NADP+ on light-induced steady-state redox changes of membrane-bound cytochromes was investigated in membrane fragements prepared from the blue-green algae Nostoc muscorum (Strain 7119) that had high rates of electron transport from water to NADP+ and from an artificial electron donor, reduced dichlorophenolindophenol (DCIPH2) to NDAP+. The membrane fragments contained very little phycocyanin and had excellent optical properties for spectrophotometric assays. With DCIPH2 as the electron donor, NADP+ had no effect on the light-induced redox changes of cytochromes: with or without NADP+, 715- or 664-nm illumination resulted mainly in the oxidation of cytochrome f and of other component(s) which may include a c-type cytochrome with an alpha peak at 549nm. With 664 nm illumination and water as the electron donor, NADP+ had a pronounced effect on the redox state of cytochromes, causing a shift toward oxidation of a component with a peak at 549 nm (possibly a c-type cytochrome), cytochrome f, and particularly cytochrome b559. Cytochrome b559 appeared to be a component of the main noncyclic electron transport chain and was photooxidized at physiological temperatures by Photosystem II. This photooxidation was apparent only in the presence of a terminal acceptor (NADP+) for the electron flow from water.

The effect of oxidation‐reduction potential on the fluorescence yield of spinach chloroplasts at 77°K

Changes of fluorescence yield in green plants have been widely assumed to be controlled directly by changes in the oxidation state of the primary electron acceptor of Photosystem II. The hypothesis of Duysens and Sweers [l] that the primary acceptor, designated Q, quenches chlorophyll fluorescence in the oxidized but not in the reduced state has been used successfully to correlate fluorescence yield changes with electrontransfer reactions. Recently, it has become apparent that the chlorophyll fluorescence yield is not solely determined by the oxidation state of the primary electron acceptor of Photosystem II. Mauzerall has demonstrated that at least six fluorescence states must be postulated to explain the fluorescence yield of Chlorelh cells over the time interval from 10e8 to lo-’ set after a light flash at room temperature [2] . Okayama and Butler have shown that in spinach chloroplasts the maximum light-induced fluorescence yield at liquid-nitrogen temperature depends on the oxidation state of cytochrome b5s9 prior to illumination [3]. Butler and co-workers have also shown [4,5] that at liquid-nitrogen temperature the kinetics of the light-induced fluorescence increase follow those of cytochrome b5s9 photooxidation rather than the kinetics of the photoreduction of the primary acceptor of Photosystem II. On the basis of these observations it has been concluded [3-61 that the fluorescence yield of Photosystem II reflects not only the oxidation state of the primary acceptor but also the oxidation state of Phso (the reaction-center chlorophyll of Photosystem II) and other components on the oxidizing side of Photosystem II. In the light of these observations, it became of interest to determine if a newly discovered

Correlation of redox levels of component electron carriers with total electron flux in an electron-transport system. P-700 and the photoreduction of NADP+ in chloroplast fragments.

A mathematical analysis is described which measures the effects of actinic light intensity and concentration of an artificial electron donor on the steady-state light-induced redox level of a reaction-center pigment (e.g. P-700) and on the overall light-induced electron flux (e.g. reduction of NADP+). The analysis led to a formulation (somewhat similar to the Michaelis-Menten equation for enzyme kinetics) in which a parameter, I1/2, is defined as the actinic light intensity that, at a given concentration of electron donro, renders the reaction-center pigment half oxidized and half reduced. To determine the role of a presumed reaction-center pigment, I1/2 is compared with another parameter, equivalent to I1/2, that is obtained independently of the reaciton-center pigment by measuring the effect of actinic light intensity and concentration of electron donor on the overall electron flow. The theory was tested and validated in a model system with spinach Photosystem I chloroplast fragments by measurements of photooxidation of P-700 and light-induced reduction of NADP+ by reduced 2,6-dichlorophenolindophenol. A possible extension of this mathematical analysis to more general electron-transport systems is discussed.

ROLE OF CYTOCHROMES AND OTHER METALLOPROTEINS IN THE PHOTOSYNTHETIC ELECTRON TRANSPORT

Publisher Summary This chapter discusses role of cytochromes and other metalloproteins in the photosynthetic electron transport. Current understanding of the mechanism of photosynthesis leans heavily on a concept of a photosynthetic, i.e., light-induced, electron transport. Photosynthetic phosphorylation (photophosphorylation) in chloroplasts is subdivided into two types, cyclic and noncyclic. The cyclic and noncyclic photophosphorylation jointly account for the basic feature of photosynthesis, i.e., conversion of radiant energy into chemical energy. The wavelength dependence of the phosphorylation associated with electron flow from an artificial electron donor (DPIPH2) to NADP resembles the cyclic system. It is well established that treating chloroplasts with ferricyanide in the dark chemically oxidizes several chloroplast constituents including cytochromes. The spectrum of the photoinduced decrease in absorbance had a maximum at 550 nm that was suggestive of an α-peak of a new cytochrome of a c-type.