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The concept of molecular mimicry describes situations in which antigen sharing between parasites and hosts could benefit pathogen evasion from host immune responses. However, antigen sharing can generate host responses to parasite-derived self-like peptides, triggering autoimmunity. Since its conception, molecular mimicry and the consequent potential cross-reactivity following infections have been repeatedly described in humans, raising increasing interest among immunologists. Here, we reviewed this concept focusing on the challenge of maintaining host immune tolerance to self-components in parasitic diseases. We focused on the studies that used genomics and bioinformatics to estimate the extent of antigen sharing between proteomes of different organisms. In addition, we comparatively analyzed human and murine proteomes for peptide sharing with proteomes of pathogenic and non-pathogenic organisms. We conclude that, although the amount of antigenic sharing between hosts and both pathogenic and non-pathogenic parasites and bacteria is massive, the degree of this antigen sharing is not related to pathogenicity or virulence. In addition, because the development of autoimmunity in response to infections by microorganisms endowed with cross-reacting antigens is rare, we conclude that molecular mimicry by itself is not a sufficient factor to disrupt intact self-tolerance mechanisms.

Macrophages carry out numerous physiological activities that are essential for both systemic and local homeostasis, as well as innate and adaptive immune responses. Their biology is intricately regulated by hormones, neuropeptides, and neurotransmitters, establishing distinct neuroendocrine axes. The control is pleiotropic, including maturation of bone marrow-derived myeloid precursors, cell differentiation into functional subpopulations, cytotoxic activity, phagocytosis, production of inflammatory mediators, antigen presentation, and activation of effector lymphocytes. Additionally, neuroendocrine components modulate macrophage ability to influence tumor growth and to prevent the spreading of infective agents. Interestingly, macrophage-derived factors enhance glucocorticoid production through the stimulation of the hypothalamic–pituitary–adrenal axis. These bidirectional effects highlight a tightly controlled balance between neuroendocrine stimuli and macrophage function in the development of innate and adaptive immune responses. Herein, we discuss how components of neuroendocrine axes impact on macrophage development and function and may ultimately influence inflammation, tissue repair, infection, or cancer progression. The knowledge of the crosstalk between macrophages and endocrine or brain-derived components may contribute to improve and create new approaches with clinical relevance in homeostatic or pathological conditions.

Abstract Helminth parasites have been studied as potential accumulators for different pollutants. Echinostoma paraensei is a foodborne trematode whose vertebrate host, the rodent Nectomys squamipes, is naturally exposed to environmental pesticides. However, little information exists regarding the pesticide’s effects on helminths. This study investigated the morphological effects on the trematode, E. paraensei, after experimental Roundup® herbicide exposure, in concentrations below those recommended for agricultural use. After two hours of exposure, scanning electron microscopy (SEM) showed changes to the tegument, such as furrowing, shrinkage, peeling, spines loss on the peristomic collar, and histopathological evidence of altered cells in the cecum and acinus vitelline glands with vacuoles and structural changes to the muscular layers. Glycidic content was decreased, primarily in the connective tissue. As E. paraensei is an intestinal parasite of the semi-aquatic wild rodent, N. squamipes, it is predisposed to pesticide exposure resulting from agricultural practices. Therefore, we emphasize the need to evaluate its impact on helminth parasites, due to their pivotal role in regulating host populations.

Tilapia is the second most farmed fish species after carp in the world. However, the production has come under threat due to emerging diseases such as tilapia lake virus (TiLV) that causes massive mortalities with high economic losses. It is largely unknown whether different tilapia strains are equally susceptible to TiLV infection. In the present study we compared the susceptibility of gray (Oreochromis niloticus x O. aureus) and red tilapia (Oreochromis spp.) to experimental TiLV infection. Virus was injected intraperitoneally at a concentration of 104 TCID50/mL. Our findings show that gray tilapia had a lower mortality, 86.44%, but statistically not significantly different (p = 0.068) from red tilapia (100%). The duration of the mortality period from onset to cessation was similar for the two species, starting at 2−3 days post challenge (dpc) with a median at 10−11 dpi and ending on 20−22 dpi. In addition, there was no difference between species in mean viral loads in brain, liver and headkidney from fish collected soon after death. As for host response, expression levels of IL-1β and TNFα were equally high in brain and headkidney samples while levels in liver samples were low for both red and gray tilapia, which coincides with lower viral loads in liver compared to brain and headkidney for both species. We find that red and gray tilapia were equally susceptible to TiLV infection with similar post challenge mortality levels, equal virus concentration in target organs and similar proinflammatory cytokine responses in target and lymphoid organs at time of death. Nonetheless, we advocate that the search for less susceptible tilapia strains should continue with the view to reduce losses from TiLV infection in aquaculture.

Herpesviruses are host specific pathogens that are widespread among vertebrates. Genome sequence data demonstrate that most herpesviruses of fish and amphibians are grouped together (family Alloherpesviridae) and are distantly related to herpesviruses of reptiles, birds and mammals (family Herpesviridae). Yet, many of the biological processes of members of the order Herpesvirales are similar. Among the conserved characteristics are the virion structure, replication process, the ability to establish long term latency and the manipulation of the host immune response. Many of the similar processes may be due to convergent evolution. This overview of identified herpesviruses of fish discusses the diseases that alloherpesviruses cause, the biology of these viruses and the host-pathogen interactions. Much of our knowledge on the biology of Alloherpesvirdae is derived from research with two species: Ictalurid herpesvirus 1 (channel catfish virus) and Cyprinid herpesvirus 3 (koi herpesvirus).

Multiple signaling pathways control every aspect of cell behavior, organ formation and tissue homeostasis throughout the lifespan of any individual. This review takes an ontogenetic view focused on the large superfamily of TGF-β/BMP ligands to address thymus morphogenesis and function in T cell differentiation. Recent findings on a role of GDF11 for reversing aging-related phenotypes are also discussed.

The aim of this article is to assemble and integrate, from a personal perspective of a research participant, seldom examined evidence that is incompatible with some basic tenets of photosynthetic electron transport, the cornerstone of which is the Z scheme. The nonconforming evidence pertaining to the mode of ferredoxin reduction and the role of the copper redox protein, plastocyanin, indicates that contrary to the Z scheme ferredoxin is reduced in two experimentally distinguishable ways: oxygenically by PS II (renamed the oxygenic photosystem), without the participation of PS I, and anoxygenically by PS I (renamed the anoxygenic photosystem). It also indicates that plastocyanin is not only, as the Z scheme asserts, the electron donor to the reaction center chlorophyll of PS I (P700) but also to the reaction center chlorophyll of PS II (P680). Other unconventional findings include evidence that the fully functional oxygenic photosystem, when operating separately from the anoxygenic photosystem, reduces plastoquinone to plastoquinol and subsequently oxidizes plastoquinol by two pathways acting in concert: one being the universally recognized DBMIB-sensitive pathway via the Rieske iron-sulfur center of the cytochrome bf complex and the other, a hitherto unrecognized, DBMIB-insensitive electron transport pathway around P680 that centers on cytochrome b-559. These nonconforming findings form the basis of an alternate hypothesis of photosynthetic electron transport that modifies and complements the Z scheme.

Historically, two main concepts guided research into possible mechanisms of light-induced atomic rearrangements in oxygenic photosynthesis: Photodecomposition of CO2 and photodecomposition of water. Both concepts envisioned photoinduced transfers of cumbersome whole atoms and not, as is currently held, photoinduced electron transfers. Early proposals for light-induced electron transfers were relegated to obscurity because they were speculative ideas, not supported by meaningful experimental findings and tied to hypothetical and ephemeral schemes. The concept of photoinduced rearrangements of whole atoms rather than electrons was so well entrenched that it was even invoked to explain their findings by the discoverers of the Hill reaction and cyclic photophosphorylation. The light-induced electron flow concept gained acceptance in photosynthesis research only with the discovery of non-cyclic photophosphorylation in which ATP formation is coupled with electron transport to ferredoxin/NADP(+) or to artificial substitutes like ferricyanide.

The carrier of photosynthetically generated reducing power is the iron-sulfur protein ferredoxin, which provides directly, or via NADP+, reducing equivalents needed for CO2 assimilation and other metabolic reactions in the cell. It is now widely held that, in oxygenic photosynthesis, the generation of reduced ferredoxin-NADP+ requires the collaboration in series of two photosystems: photosystem II (PSII), which energizes electrons to an intermediate reducing potential and transfers them to photosystem I (PSI), which in turn is solely competent to energize electrons to the strong reducing potential required for the reduction of ferredoxin-NADP+ (the Z scheme). This investigation tested the premise of an alternative scheme, which envisions that PSII, without the involvement of PSI, is also capable of photoreducing ferredoxin-NADP+. We report here unexpected findings consistent with the alternative scheme. Isolated PSII reaction centers (completely free of PSI components), when supplemented with ferredoxin, ferredoxin-NADP+ oxidoreductase, and a PSII electron donor,1,5-diphenylcarbazide, gave a significant photoreduction of NADP+. A striking feature of this electron transfer from a PSII donor to the perceived terminal acceptor of PSI was its total dependence on catalytic quantities of plastocyanin, a copper-containing electron-transport protein hitherto known only as an electron donor to PSI.

The quantum efficiency of photosynthetic energy conversion was investigated in isolated spinach chloroplasts by measurements of the quantum requirements of ATP formation by cyclic and noncyclic photophosphorylation catalyzed by ferredoxin. ATP formation had a requirement of about 2 quanta per 1 ATP at 715 nm (corresponding to a requirement of 1 quantum per electron) and a requirement of 4 quanta per ATP (corresponding to a requirement of 2 quanta per electron) at 554 nm. When cyclic and noncyclic photophosphorylation were operating concurrently at 554 nm, a total of about 12 quanta was required to generate the two NADPH and three ATP needed for the assimilation of one CO2 to the level of glucose.

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.

Acetylene reduction by nitrogenase from Rhodospirillum rubrum, unlike that by other nitrogenases, was recently found by other investigators to require an activation of the iron protein of nitrogenase by an activating system comprising a chromatophore membrane component, adenosine 5'-triphosphate (ATP), and divalent metal ions. In an extension of this work, we observed that the same activating system was also required for nitrogenase-linked H(2) evolution. However, we found that, depending on their nitrogen nutrition regime, R. rubrum cells produced two forms of nitrogenase that differed in their Fe protein components. Cells whose nitrogen supply was totally exhausted before harvest yielded predominantly a form of nitrogenase (A) whose enzymatic activity was not governed by the activating system, whereas cells supplied up to harvest time with N(2) or glutamate yielded predominantly a form of nitrogenase (R) whose enzymatic activity was regulated by the activating system. An unexpected finding was the rapid (less than 10 min in some cases) intracellular conversion of nitrogenase A to nitrogenase R brought about by the addition to nitrogen-starved cells of glutamine, asparagine, or, particularly, ammonia. This finding suggests that mechanisms other than de novo protein synthesis were involved in the conversion of nitrogenase A to the R form. The molecular weights of the Fe protein and Mo-Fe protein components from nitrogenases A and R were the same. However, nitrogenase A appeared to be larger in size, because it had more Fe protein units per Mo-Fe protein than did nitrogenase R. A distinguishing property of the Fe protein from nitrogenase R was its ATP requirement. When combined with the Mo-Fe protein (from either nitrogenase A or nitrogenase R), the R form of Fe protein required a lower ATP concentration but bound or utilized more ATP molecules during acetylene reduction than did the A form of Fe protein. No differences between the Fe proteins from the two forms of nitrogenase were found in the electron paramagnetic resonance spectrum, midpoint oxidation-reduction potential, or sensitivity to iron chelators.

Electron paramagnetic resonance spectrometry was used to investigate, at physiological temperatures, light-induced electron transport from membrane-bound iron-sulfur components (bound ferredoxin) to soluble ferredoxin and NADP(+) in membrane fragments (from the blue-green alga, Nostoc muscorum) that had high rates of electron transport from water to NADP(+) and from an artificial electron donor, reduced dichlorophenolindophenol (DCIPH(2)) to NADP(+). Illumination at 20 degrees resulted in the photoreduction of membrane-bound iron-sulfur centers A and B. Photoreduction by water gave electron paramagnetic resonance signals of both centers A and B; photoreduction by DCIPH(2) was found to generate a strong electron paramagnetic signal of only center B. When water was the reductant, the addition and photoreduction of soluble ferredoxin generated additional signals characteristics of soluble ferredoxin without causing a decrease in the amplitude of the signals due to centers A and B. The further addition of NADP(+) (and its photoreduction) greatly diminished signals due to the bound iron-sulfur centers and to soluble ferredoxin. An outflow of electrons from center B to soluble ferredoxin and NADP(+) was particularly pronounced when DCIPH(2) was the reductant. These observations provide the first evidence for a light-induced electron transport between membrane-bound iron-sulfur centers and ferredoxin-NADP(+). The relationship of these observations to current concepts of photosynthetic electron transport is discussed.

An investigation by paramagnetic resonance spectroscopy of the photoreduction of ferredoxin, oxygenically by water and anoxygenically by a direct electron donor to photosystem I, led to the unexpected findings that different reductive mechanisms may be involved. Ferredoxin photoreduced by water was not reoxidized in the light under aerobic conditions and, under anaerobic conditions, it was remarkably resistant to reoxidation in the dark. By contrast, ferredoxin photoreduced by a donor to photosystem I was readily reoxidized in the light by air and, under anaerobic conditions, by exposure to darkness. Furthermore, when electron transport linking photosystems I and II was inhibited by a plastoquinone antagonist, ferredoxin was photoreduced by water with no evidence for an accompanying photoreduction of the more electronegative bound iron-sulfur centers in chloroplasts. These findings are at variance with the now prevalent concepts of photosynthetic electron transport.

New evidence is presented in support of the concept that reducing power for photosynthesis is generated solely by photosystem II (the oxygenic photosystem) when it transfers electrons from water to ferredoxin without the collaboration of photosystem I, the anoxygenic photosystem responsible for cyclic photophosphorylation. Membrane vesicles of opposite sidedness were prepared from spinach chloroplasts by the two-phase partition method: inside-out-vesicles greatly enriched in photosystem II and right-side-out vesicles containing both photosystems and having the same sidedness orientation as unfractionated chloroplast membranes. In both types of vesicles, plastoquinone analogues were used to inhibit light-induced electron transport from water to ferredoxin and from water to native photosystem I acceptors, the membrane-bound iron-sulfur centers A and B. In right-side-out vesicles the photoreduction of iron-sulfur centers A and B was more sensitive to plastoquinone inhibitors than the photoreduction of ferredoxin, whereas the converse was found in inside-out vesicles in which a greatly enhanced sensitivity of ferredoxin reduction to plastoquinone inhibitors was detected: the photoreduction of ferredoxin was about 80% inhibited at low concentrations of plastoquinone inhibitors that had practically no effect on the photoreduction of iron-sulfur centers A and B. These findings appear to exclude the possibility that these photosystem I contaminants were involved in the photoreduction of ferredoxin by the PSII-enriched inside-out vesicles.

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.

Although cytochrome b-559 has long been known as a membrane-bound redox component closely linked to the reaction center of the oxygen-generating photosystem (PSII), its role in photosynthesis has remained obscure. This paper reports evidence and outlines a hypothesis in support of a "b-559 cycle"-i.e., a light-induced, cytochrome b-559-dependent, cyclic electron transport pathway around PSII that promotes translocation of protons from plastoquinol into the aqueous domain (lumen) of photosynthetic membranes (thylakoids). Light-induced proton transport coupled to light-induced electron transport is an essential aspect of energy transduction in photosynthesis because it generates an electrochemical proton gradient that drives ATP synthesis by the process of photosynthetic phosphorylation. The principal carrier of electrons and protons in thylakoids is the plastoquinone/plastoquinol couple. We propose that the b-559 cycle functions as a redox-linked proton pump that may operate jointly with the Rieske iron-sulfur pathway in oxidizing plastoquinol. The overall effect of such concerted oxidation of plastoquinol would be the translocation into the thylakoid lumen of two protons for each electron transferred from water to plastocyanin via plastoquinone.

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.

It is now widely held that the light-induced noncyclic (linear) electron transport from water to NADP(+) requires the collaboration in series of the two photosystems that operate in oxygen-evolving cells: photosystem II (PSII) photooxidizes water and transfers electrons to photosystem I (PSI); PSI photoreduces ferredoxin, which in turn reduces NADP(+) (the Z scheme). However, a recently described alternative scheme envisions that PSII drives the noncyclic electron transport from water to ferredoxin and NADP(+) without the collaboration of PSI, whose role is limited to cyclic electron transport [Arnon, D. I., Tsujimoto, H. Y. & Tang, G. M.-S. (1981) Proc. Natl. Acad. Sci. USA 78, 2942-2946]. Reported here are findings at variance with the Z scheme and consistent with the alternative scheme. Thylakoid membrane vesicles were isolated from spinach chloroplasts by the two-phase aqueous polymer partition method. Vesicles, originating mainly from appressed chloroplast membranes that are greatly enriched in PSII, were turned inside-out with respect to the original sidedness of the membrane. With added plastocyanin, ferredoxin, and ferredoxin-NADP(+) reductase, the inside-out vesicles enriched in PSII gave a significant photoreduction of NADP(+) with water as electron donor, under experimental conditions that appear to exclude the participation of PSI.

Under aerobic conditions that are likely to prevail in chloroplasts in vivo, the optimal concentration of ferredoxin for cyclic photophosphorylation was found to be equal to that required for NADP reduction and about one-tenth of that needed for cyclic photophosphorylation under anaerobic conditions. In the presence of ferredoxin and NADP, cyclic photophosphorylation operated concurrently with noncyclic photophosphorylation, producing an ATP: NADPH ratio of about 1.5. The effective operation of ferredoxin-catalyzed cyclic photophosphorylation by itself required a curtailment of the electron flow from water which was accomplished experimentally by the use of either an inhibitor or far-red monochromatic light. An unexpected discovery was that the operation of cyclic photophosphorylation by itself was also regulated by a back reaction of NADPH and ferredoxin with two components of chloroplast membranes, component C550 and cytochrome b559. The significance of these findings to photosynthesis in vivo is discussed.

The photosynthetic apparatus converts light into chemical energy by a series of reactions that give rise to a coupled flow of electrons and protons that generate reducing power and ATP, respectively. A key intermediate in these reactions is plastoquinone (PQ), the most abundant electron and proton (hydrogen) carrier in photosynthetic membranes (thylakoids). PQ ultimately transfers electrons to a terminal electron acceptor by way of the Rieske Fe-S center of the cytochrome bf complex. In the absence of a terminal acceptor, electrons accumulate in the PQ pool, which is reduced to plastoquinol (PQH(2)), and also on a specialized PQ, Q(A), which is reduced to an unprotonated semiquinone anion (Q(A) (-)). The accumulation of Q(A) (-) is measured by a rise in fluorescence yield and the accumulation of PQH(2) is measured by absorption difference spectrometry. We have found that in the absence of a terminal electron acceptor, two chemically diverse proton-conducting ionophores (protonophores), 2,6-di-t-butyl-4-(2',2'-dicyanovinyl)phenol (SF 6847) and carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP), induced oxidation of PQH(2) and quenching of chloroplast fluorescence, signifying oxidation of Q(A) (-). The two protonophores produced the same effects even when the only recognized pathway of PQH(2) oxidation by way of the cytochrome bf complex was inhibited by dibromothymoquinone. Two other uncouplers, gramicidin and nigericin, which are not protonophores but facilitate proton movement across membranes by other mechanisms, were ineffective. These findings are consistent with the operation in the oxygen-generating photosystem (photosystem II) of a cyclic, proton-conducting pathway.

A new iron-sulfur protein, distinct from the soluble chloroplast ferredoxin, was isolated from chloroplast membranes. The isolated protein, purified to homogeneity, had a molecular weight of about 8000 and 4 atoms of iron and 4 inorganic sulfides per mole. Its absorption spectrum had a broad absorbance band in the 400 nm region, a shoulder at approximately 310 nm, and a peak around 280 nm. The absorbance ratio A(400) to A(280) was 0.55. The electron paramagnetic resonance spectrum (measured at 12 degrees K) of the reduced protein was similar to that of other reduced iron-sulfur proteins, showing a major resonance line at g = 1.94. The isolated protein, when photoreduced by spinach chloroplasts, can in turn transfer electrons to mammalian cytochrome c. However, the photoreduced protein cannot replace soluble ferredoxin in NADP(+) reduction because of its apparent inability to interact with the chloroplast enzyme, ferredoxin-NADP(+) reductase. The relation of the isolated iron-sulfur protein to the bound ferredoxin that acts as the primary electron acceptor in Photosystem I is discussed.

In coupling the reducing power of illuminated chloroplasts to the nitrogenase from photosynthetically grown Rhodospirillum rubrum cells, one of the native ferredoxins. FdI, was found to be three times more effective than FdII.

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.

The currently prevalent concept of the generation of photosynthetic reducing power in oxygen-evolving cells envisions a linear (noncyclic) electron flow from water to ferredoxin (and thence to NADP(+)) that requires the collaboration of photosystems I and II (PSI and PSII) joined by plastoquinone and other electron carriers (the Z scheme). The essence of the Z scheme is that only PSI can reduce ferredoxin-i.e., that, after being energized to an intermediate reducing potential by PSII, electrons from water are transported via plastoquinone to PSI which energizes the electrons to their ultimate reducing potential adequate for the reduction of ferredoxin. Basic to the Z scheme is the function of plastoquinone as the obligatory link in electron transport from PSII to PSI. However, we have found that, when plastoquinone function was inhibited, ferredoxin was photoreduced by water without the collaboration of PSI. We now report evidence for an important function of plastoquinone in the translocation of protons liberated inside the thylakoid membrane by photooxidation of water. When the oxygenic photoreduction (i.e., by water) of ferredoxin was blocked by plastoquinone inhibitors, dibromothymoquinone or dinitrophenol ether of iodonitrothymol, the photoreduction of ferredoxin was restored by each of four chemically diverse uncouplers, similar only in their ability to facilitate proton movement across membranes. Similar results were obtained for the oxygenic reduction of NADP(+). Our results suggest that the light-induced electron flow from water cannot be maintained unless the simultaneously liberated protons are removed from inside the membrane via plastoquinone. The new evidence is embodied in a concept of an oxygenic photosystem for photosynthetic electron and proton transport, which we propose as an alternative to the Z scheme, to account for photoreduction of ferredoxin-NADP(+) by water and the coupled oxygenic (formerly noncyclic) ATP formation without involving PSI. The role of the anoxygenic photosystem (formerly called PSI) is ATP formation by cyclic photophosphorylation.

Other investigators have isolated soluble, low-potential, c-type cytochromes (cytochrome c3) from a few photosynthetic procaryotes, i.e., a cyanobacterium and two species of purple nonsulfur bacteria. However, such cytochromes appeared to be absent from other purple bacteria, including Rhodospirillum rubrum and Chromatium vinosum. We now report evidence for the presence of low-potential c-type cytochromes in these two species, in which they were found to be bound to the photosynthetic membranes. Evidence for a membrane-bound, low-potential c-type cytochrome was also found in Rhodopseudomonas sphaeoides. The low-potential c-type cytochrome of R. rubrum was solubilized by a Triton X-100 treatment of chromatophores and was partly purified. It was found to have a molecular weight of about 17,000, a midpoint oxidation-reduction potential of -192 mV, and an alpha-absorption peak at 552 nm. It appears that low-potential c-type cytochromes may be present in all purple photosynthetic bacteria, of both the sulfur and the nonsulfur types.

Previous reports from this laboratory described a new concept of three light reactions in plant photosynthesis comprising two short-wavelength (lambda < 700 nm) photoreactions belonging to Photosystem II and one long-wavelength (lambda > 700 nm) photoreaction belonging to Photosystem I. Among the electron carriers assigned to Photosystem II were cytochrome b(559) and plastocyanin and to Photosystem I, cytochrome f.According to a widely held view, the light-induced reduction of NADP by water requires the collaboration of Photosystems I and II and involves specifically cytochrome f and P700 (a portion of chlorophyll a peculiar to Photosystem I). By contrast, the new concept ascribes the light-induced reduction of NADP by water solely to the two photoreactions of Photosystem II, without the participation of Photosystem I and its components, cytochrome f and P700.Further evidence in support of the new concept has now been obtained from chloroplast fragments. Two kinds of chloroplast fragments have been prepared: (a) one with Photosystem II activity, capable-in the presence of plastocyanin-of photoreducing NADP with water but lacking P700 and functional cytochrome f and (b) another having only Photosystem I activity, lacking plastocyanin, and enriched in P700.