Healthcare personnel intestinal colonization with multidrug-resistant organisms

OBJECTIVES

This study aims to assess the association between patient contact and intestinal carriage of multidrug-resistant organisms (MDRO) by sampling healthcare personnel (HCP) and staff without patient contact.

METHODS

For this observational study, we recruited 400 HCP who worked in our 200-bed research hospital and 400 individuals without patient contact between November 2013 and February 2015. Participants submitted two self-collected perirectal swabs and a questionnaire. Swabs were processed for multidrug-resistant Gram-negative bacteria and vancomycin-resistant enterococci (VRE). Questionnaires explored occupational and personal risk factors for MDRO carriage.

RESULTS

Among 800 participants, 94.4% (755/800) submitted at least one swab, and 91.4% (731/800) also submitted questionnaires. Extended spectrum β-lactamase-producing organisms were recovered from 3.4% (26/755) of participants, and only one carbapenemase-producing organism was recovered. No VRE were detected. The potential exposure of 68.9% (250/363) of HCP who reported caring for MDRO-colonized patients did not result in a rate of MDRO carriage among HCP (4.0%; 15/379) significantly higher than that of staff without patient contact (3.2%; 12/376; p 0.55).

CONCLUSIONS

This is the largest US study of HCP intestinal MDRO carriage. The low colonization rate is probably reflective of local community background rates, suggesting that HCP intestinal colonization plays a minor role in nosocomial spread of MDROs in a non-outbreak setting.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01952158.

Persistence of Pseudomonas aeruginosa in a pulmonary nodule with late relapse

Lung nodules are common diagnostic challenges in hematopoietic stem cell transplantation and solid organ transplantation. Pseudomonas aeruginosa is a known cause of lung abscess in these patients, but its ability to persist for months in a quiescent lung nodule and later cause recurrent infection is not well known or documented. A patient with a history of acute pre-B-cell lymphoblastic leukemia had enlargement and cavitation of a small right upper lobe pulmonary nodule 10 months after allogeneic hematopoietic stem cell transplantation. The nodule was the remnant of a presumed P. aeruginosa septic embolus that occurred 2.5 months after transplantation. With antibiotic treatment, the nodule had shrunk in size to <1 cm and remained stable. Transthoracic needle aspiration grew P. aeruginosa indistinguishable by molecular typing from isolates obtained 7.5 months earlier from blood and bronchoalveolar lavage fluid. Sub-centimeter pulmonary nodules attributable to previously treated P. aeruginosa may harbor viable organisms and lead to recrudescent infection.

Promising new assays and technologies for the diagnosis and management of infectious diseases

In the first decade of the 21st century, we have seen the completion of the human genome project and marked progress in the human microbiome project. The vast amount of data generated from these efforts combined with advances in molecular and biomedical technologies have led to the development of a multitude of assays and technologies that may be useful in the diagnosis and management of infectious diseases. Here, we identify several new assays and technologies that have recently come into clinical use or have potential for clinical use in the near future. The scope of this review is broad and includes topics such as the serum marker procalcitonin, gene expression profiling, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and nucleic acid aptamers. Principles that underlie each assay or technology, their clinical applications, and potential strengths and limitations are addressed.

Charge separation is virtually irreversible in photosystem II core complexes with oxidized primary quinone acceptor

X-ray structures of the Photosystem II (PSII) core revealed relatively large interpigment distances between the CP43 and CP47 antenna complexes and the reaction center (RC) with respect to the interpigment distances in a single unit. This finding questions the possibility of fast energy equilibration among the antenna and the RC, which has been the basic explanation for the measured PSII fluorescence kinetics for more than two decades. In this study, we present time-resolved fluorescence measurements obtained with a streak-camera setup on PSII core complexes from Thermosynechococcus elongatus at room temperature (RT) and at 77 K. Kinetic modeling of the RT data obtained with oxidized quinone acceptor Q(A), reveals that the kinetics are best described by fast primary charge separation at a time scale of 1.5 ps and slow energy transfer from the antenna into the RC, which results in an energy equilibration time between the antenna and the RC of about 44 ps. This model is consistent with structure-based computations. Primary radical pair formation was found to be a virtually irreversible process. Energy equilibration within the CP43 and CP47 complexes is shown to occur at a time scale of 8 ps. Kinetic modeling of the 77 K data reveals similar energy transfer time scales in the antenna units and among the antenna and the RC as at RT, respectively, 7 and 37 ps. We conclude that the energy transfer from the CP43/CP47 antenna to the RC is the dominant factor in the total charge separation kinetics in intact PSII cores.

Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy

In this work, the transfer of excitation energy was studied in native and cation-depletion induced, unstacked thylakoid membranes of spinach by steady-state and time-resolved fluorescence spectroscopy. Fluorescence emission spectra at 5 K show an increase in photosystem I (PSI) emission upon unstacking, which suggests an increase of its antenna size. Fluorescence excitation measurements at 77 K indicate that the increase of PSI emission upon unstacking is caused both by a direct spillover from the photosystem II (PSII) core antenna and by a functional association of light-harvesting complex II (LHCII) to PSI, which is most likely caused by the formation of LHCII-LHCI-PSI supercomplexes. Time-resolved fluorescence measurements, both at room temperature and at 77 K, reveal differences in the fluorescence decay kinetics of stacked and unstacked membranes. Energy transfer between LHCII and PSI is observed to take place within 25 ps at room temperature and within 38 ps at 77 K, consistent with the formation of LHCII-LHCI-PSI supercomplexes. At the 150-160 ps timescale, both energy transfer from LHCII to PSI as well as spillover from the core antenna of PSII to PSI is shown to occur at 77 K. At room temperature the spillover and energy transfer to PSI is less clear at the 150 ps timescale, because these processes compete with charge separation in the PSII reaction center, which also takes place at a timescale of about 150 ps.

Pigment spectra and intermolecular interaction potentials in glasses and proteins

A model is proposed for chromophore optical spectra in solids over a wide range of temperatures and pressures. Inhomogeneous band shapes and their pressure dependence, as well as baric shift coefficients of spectral lines, selected by the frequency, were derived using Lennard-Jones potentials of the ground and excited states. Quadratic electron-phonon coupling constants, describing the thermal shift and broadening of zero-phonon lines, were also calculated. Experimentally, thermal shift and broadening of spectral holes were studied between 5 and 40 K for a synthetic pigment, chlorin, embedded in polymer hosts. The baric effects on holes were determined by applying hydrostatic He gas pressure up to 200 bar, at 6 K. Absorption spectra of pheophytin a, chlorophyll a, and beta-carotene in polymers and plant photosystem II CP47 complex were measured between 5 (or 77) and 300 K, and subject to Voigtian deconvolution. A narrowing of inhomogeneous bandwidth with increasing temperature, predicted on the basis of hole behavior, was observed as the shrinking of Gaussian spectral component. The Lorentzian broadening was ascribed to optical dephasing up to 300 K in transitions with weak to moderate linear electron-phonon coupling strength. The thermal broadening is purely Gaussian in multiphonon transitions (S(2) band of beta-carotene, Soret bands of tetrapyrrolic pigments), and the Lorentz process appears to be suppressed, indicating a lack of exponential dephasing. Density, polarity, polarizability, compressibility, and other local parameters of the pigment binding sites in biologically relevant systems can be deduced from spectroscopic data, provided that sufficient background information is available.

Excitation energy transfer pathways in Lhca4

EET in reconstituted Lhca4, a peripheral light-harvesting complex from Photosystem I of Arabidopsis thaliana, containing 10 chlorophylls and 2 carotenoids, was studied at room temperature by femtosecond transient absorption spectroscopy. Two spectral forms of Lut were observed in the sites L1 and L2, characterized by significantly different interactions with nearby chlorophyll a molecules. A favorable interpretation of these differences is that the efficiency of EET to Chls is about two times lower from the "blue" Lut in the site L1 than from the "red" Lut in the site L2 due to fast IC in the former case. A major part of the energy absorbed by the "red" Lut, approximately 60%-70%, is transferred to Chls on a sub-100-fs timescale from the state S(2) but, in addition, minor EET from the hot S(1) state within 400-500 fs is also observed. EET from the S(1) state to chlorophylls occurs also within 2-3 ps and is ascribed to Vio and/or "blue" Lut. EET from Chl b to Chl a is biphasic and characterized by time constants of approximately 300 fs and 3.0 ps. These rates are ascribed to EET from Chl b spectral forms absorbing at approximately 644 nm and approximately 650 nm, respectively. About 25% of the excited Chls a decays very fast-within approximately 15 ps. This decay is proposed to be related to the presence of the interacting Chls A5 and B5 located next to the carotenoid in the site L2 and may imply some photoprotective role for Lhca4 in the photosystem I super-complex.

Energy and electron transfer in photosystem II reaction centers with modified pheophytin composition

Energy and electron transfer in Photosystem II reaction centers in which the photochemically inactive pheophytin had been replaced by 13(1)-deoxo-13(1)-hydroxy pheophytin were studied by femtosecond transient absorption-difference spectroscopy at 77 K and compared to the dynamics in untreated reaction center preparations. Spectral changes induced by 683-nm excitation were recorded both in the Q(Y) and in the Q(X) absorption regions. The data could be described by a biphasic charge separation. In untreated reaction centers the major component had a time constant of 3.1 ps and the minor component 33 ps. After exchange, time constants of 0.8 and 22 ps were observed. The acceleration of the fast phase is attributed in part to the redistribution of electronic transitions of the six central chlorin pigments induced by replacement of the inactive pheophytin. In the modified reaction centers, excitation of the lowest energy Q(Y) transition produces an excited state that appears to be localized mainly on the accessory chlorophyll in the active branch (B(A) in bacterial terms) and partially on the active pheophytin H(A). This state equilibrates in 0.8 ps with the radical pair. B(A) is proposed to act as the primary electron donor also in untreated reaction centers. The 22-ps (pheophytin-exchanged) or 33-ps (untreated) component may be due to equilibration with the secondary radical pair. Its acceleration by H(B) exchange is attributed to a faster reverse electron transfer from B(A) to. After exchange both and are nearly isoenergetic with the excited state.

Plants lacking the main light-harvesting complex retain photosystem II macro-organization

Photosystem II (PSII) is a key component of photosynthesis, the process of converting sunlight into the chemical energy of life. In plant cells, it forms a unique oligomeric macrostructure in membranes of the chloroplasts. Several light-harvesting antenna complexes are organized precisely in the PSII macrostructure-the major trimeric complexes (LHCII) that bind 70% of PSII chlorophyll and three minor monomeric complexes-which together form PSII supercomplexes. The antenna complexes are essential for collecting sunlight and regulating photosynthesis, but the relationship between these functions and their molecular architecture is unresolved. Here we report that antisense Arabidopsis plants lacking the proteins that form LHCII trimers have PSII supercomplexes with almost identical abundance and structure to those found in wild-type plants. The place of LHCII is taken by a normally minor and monomeric complex, CP26, which is synthesized in large amounts and organized into trimers. Trimerization is clearly not a specific attribute of LHCII. Our results highlight the importance of the PSII macrostructure: in the absence of one of its main components, another protein is recruited to allow it to assemble and function.

Supermolecular organization of photosystem II and its associated light-harvesting antenna in Arabidopsis thaliana

The organization of Arabidopsis thaliana photosystem II (PSII) and its associated light-harvesting antenna (LHCII) was studied in isolated PSII-LHCII supercomplexes and native membrane-bound crystals by transmission electron microscopy and image analysis. Over 4000 single-particle projections of PSII-LHCII supercomplexes were analyzed. In comparison to spinach supercomplexes [Boekema, E.J., van Roon, H., van Breemen, J.F.L. & Dekker, J.P. (1999) Eur. J. Biochem. 266, 444-452] some striking differences were revealed: a much larger number of supercomplexes from Arabidopsis contain copies of M-type LHCII trimers. M-type trimers can also bind in the absence of the more common S-type trimers. No binding of l-type trimers could be detected. Analysis of native membrane-bound PSII crystals revealed a novel type of crystal with a unit cell of 25.6 x 21.4 nm (angle 77 degrees ), which is larger than any of the PSII lattices observed before. The data show that the unit cell is built up from C2S2M2 supercomplexes, rather than from C2S2M supercomplexes observed in native membrane crystals from spinach [Boekema, E.J., Van Breemen, J.F.L., Van Roon, H. & Dekker, J.P. (2000) J. Mol. Biol. 301, 1123-1133]. It is concluded from both the single particle analysis and the crystal analysis that the M-type trimers bind more strongly to PSII core complexes in Arabidopsis than in spinach.

Alloying effects on electromigration mass transport

Small amounts of alloying elements can significantly retard electromigration in conductor lines. This phenomenon is experimentally well established but is still lacking a fundamental explanation. An atomic-level mechanism for this behavior is proposed here which is based on a kinetic analysis of diffusion in crystalline interfaces. It predicts a reduction or reversal of the flux of host atoms for physically reasonable parameters and can account for the observed effect of copper on electromigration in aluminum conductor lines.

Time-resolved fluorescence emission measurements of photosystem I particles of various cyanobacteria: a unified compartmental model

Photosystem I (PS-I) contains a small fraction of chlorophylls (Chls) that absorb at wavelengths longer than the primary electron donor P700. The total number of these long wavelength Chls and their spectral distribution are strongly species dependent. In this contribution we present room temperature time-resolved fluorescence data of five PS-I core complexes that contain different amounts of these long wavelength Chls, i.e., monomeric and trimeric photosystem I particles of the cyanobacteria Synechocystis sp. PCC 6803, Synechococcus elongatus, and Spirulina platensis, which were obtained using a synchroscan streak camera. Global analysis of the data reveals considerable differences between the equilibration components (3.4-15 ps) and trapping components (23-50 ps) of the various PS-I complexes. We show that a relatively simple compartmental model can be used to reproduce all of the observed kinetics and demonstrate that the large kinetic differences are purely the result of differences in the long wavelength Chl content. This procedure not only offers rate constants of energy transfer between and of trapping from the compartments, but also well-defined room temperature emission spectra of the individual Chl pools. A pool of red shifted Chls absorbing around 702 nm and emitting around 712 nm was found to be a common feature of all studied PS-I particles. These red shifted Chls were found to be located neither very close to P700 nor very remote from P700. In Synechococcus trimeric and Spirulina monomeric PS-I cores, a second pool of red Chls was present which absorbs around 708 nm, and emits around 721 nm. In Spirulina trimeric PS-I cores an even more red shifted second pool of red Chls was found, absorbing around 715 nm and emitting at 730 nm.

Green plant photosystem I binds light-harvesting complex I on one side of the complex

We report a structural characterization by electron microscopy of green plant photosystem I solubilized by the mild detergent n-dodecyl-alpha-D-maltoside. It is shown by immunoblotting that the isolated complexes contain all photosystem I core proteins and all peripheral light-harvesting proteins. The electron microscopic analysis is based on a large data set of 14 000 negatively stained single-particle projections and reveals that most of the complexes are oval-shaped monomers. The monomers have a tendency to associate into artificial dimers, trimers, and tetramers in which the monomers are oppositely oriented. Classification of the dimeric complexes suggests that some of the monomers lack a part of the peripheral antenna. On the basis of a comparison with projections from trimeric photosystem I complexes from cyanobacteria, we conclude that light-harvesting complex I only binds to the core complex at the side of the photosystem I F/J subunits and does not cause structural hindrances for the type of trimerization observed in cyanobacterial photosystem I.

Conformational changes in photosystem II supercomplexes upon removal of extrinsic subunits

Photosystem II is a multisubunit pigment-protein complex embedded in the thylakoid membranes of chloroplasts. It consists of a large number of intrinsic membrane proteins involved in light-harvesting and electron-transfer processes and of a number of extrinsic proteins required to stabilize photosynthetic oxygen evolution. We studied the structure of dimeric supercomplexes of photosystem II and its associated light-harvesting antenna by electron microscopy and single-particle image analysis. Comparison of averaged projections from native complexes and complexes without extrinsic polypeptides indicates that the removal of 17 and 23 kDa extrinsic subunits induces a shift of about 1.2 nm in the position of the monomeric peripheral antenna protein CP29 toward the central part of the supercomplex. Removal of the 33 kDa extrinsic protein induces an inward shift of the strongly bound trimeric light-harvesting complex II (S-LHCII) of about 0.9 nm, and in addition destabilizes the monomer-monomer interactions in the central core dimer, leading to structural rearrangements of the core monomers. It is concluded that the extrinsic subunits keep the S-LHCII and CP29 subunits in proper positions at some distance from the central part of the photosystem II core dimer to ensure a directed transfer of excitation energy through the monomeric peripheral antenna proteins CP26 and CP29 and/or to maintain sequestered domains of inorganic cofactors required for oxygen evolution.

Arrangement of photosystem II supercomplexes in crystalline macrodomains within the thylakoid membrane of green plant chloroplasts

The chloroplast thylakoid membrane of green plants is organized in stacked grana membranes and unstacked stroma membranes. We investigated the structural organization of Photosystem II (PSII) in paired grana membrane fragments by transmission electron microscopy. The membrane fragments were obtained by a short treatment of thylakoid membranes with the mild detergent n-dodecyl-alpha, d-maltoside and are thought to reflect the grana membranes in a native state. The membranes frequently show crystalline macrodomains in which PSII is organized in rows spaced by either 26.3 nm (large-spaced crystals) or 23 nm (small-spaced crystals). The small-spaced crystals are less common but better ordered. Image analysis of the crystals by an aperiodic approach revealed the precise positions of the core parts of PSII in the lattices, as well as features of the peripheral light-harvesting antenna. Together, they indicate that the so-called C(2)S(2) and C(2)S(2)M supercomplexes form the basic motifs of the small-spaced and large-spaced crystals, respectively. An analysis of a pair of membranes with a well-ordered large-spaced crystal reveals that many PSII complexes in one layer face only light-harvesting complexes (LHCII) in the other layer. The implications of this type of organization for the efficient transfer of excitation energy from LHCII to PSII and for the stacking of grana membranes are discussed.

Evidence for two spectroscopically different dimers of light-harvesting complex I from green plants

A preparation consisting of isolated dimeric peripheral antenna complexes from green plant photosystem I (light-harvesting complex I or LHCI) has been characterized by means of (polarized) steady-state absorption and fluorescence spectroscopy at low temperatures. We show that this preparation can be described reasonably well by a mixture of two types of dimers. In the first dimer about 10% of all Q(y)() absorption of the chlorophylls arises from two chlorophylls with absorption and emission maxima at about 711 and 733 nm, respectively, whereas in the second about 10% of the absorption arises from two chlorophylls with absorption and emission maxima at about 693 and 702 nm, respectively. The remaining chlorophylls show spectroscopic properties comparable to those of the related peripheral antenna complexes of photosystem II. We attribute the first dimer to a heterodimer of the Lhca1 and Lhca4 proteins and the second to a hetero- or homodimer of the Lhca2 and/or Lhca3 proteins. We suggest that the chlorophylls responsible for the 733 nm emission (F-730) and 702 nm emission (F-702) are excitonically coupled dimers and that F-730 originates from one of the strongest coupled pair of chlorophylls observed in nature.

Solubilization of green plant thylakoid membranes with n-dodecyl-alpha,D-maltoside. Implications for the structural organization of the Photosystem II, Photosystem I, ATP synthase and cytochrome b6 f complexes

A biochemical and structural analysis is presented of fractions that were obtained by a quick and mild solubilization of thylakoid membranes from spinach with the non-ionic detergent n-dodecyl-alpha,D-maltoside, followed by a partial purification using gel filtration chromatography. The largest fractions consisted of paired, appressed membrane fragments with an average diameter of about 360 nm and contain Photosystem II (PS II) and its associated light-harvesting antenna (LHC II), but virtually no Photosystem I, ATP synthase and cytochrome b (6) f complex. Some of the membranes show a semi-regular ordering of PS II in rows at an average distance of about 26.3 nm, and from a partially disrupted grana membrane fragment we show that the supercomplexes of PS II and LHC II represent the basic structural unit of PS II in the grana membranes. The numbers of free LHC II and PS II core complexes were very high and very low, respectively. The other macromolecular complexes of the thylakoid membrane occurred almost exclusively in dispersed forms. Photosystem I was observed in monomeric or multimeric PS I-200 complexes and there are no indications for free LHC I complexes. An extensive analysis by electron microscopy and image analysis of the CF(0)F(1) ATP synthase complex suggests locations of the delta (on top of the F(1) headpiece) and in subunits (in the central stalk) and reveals that in a substantial part of the complexes the F(1) headpiece is bended considerably from the central stalk. This kinking is very likely not an artefact of the isolation procedure and may represent the complex in its inactive, oxidized form.

Primary charge separation in Photosystem II

In this Minireview, we discuss a number of issues on the primary photosynthetic reactions of the green plant Photosystem II. We discuss the origin of the 683 and 679 nm absorption bands of the PS II RC complex and suggest that these forms may reflect the single-site spectrum with dominant contributions from the zero-phonon line and a pronounced approximately 80 cm(-1) phonon side band, respectively. The couplings between the six central RC chlorins are probably very similar and, therefore, a 'multimer' model arises in which there is no 'special pair' and in which for each realization of the disorder the excitation may be dynamically localized on basically any combination of neighbouring chlorins. The key features of our model for the primary reactions in PS II include ultrafast (<500 fs) energy transfer processes within the multimer, 'slow' ( approximately 20 ps) energy transfer processes from peripheral RC chlorophylls to the RC multimer, ultrafast charge separation (<500 fs) with a low yield starting from the singlet-excited 'accessory' chlorophyll of the active branch, cation transfer from this 'accessory' chlorophyll to a 'special pair' chlorophyll and/or charge separation starting from this 'special pair' chlorophyll ( approximately 8 ps), and slow relaxation ( approximately 50 ps) of the radical pair by conformational changes of the protein. The charge separation in the PS II RC can probably not be described as a simple trap-limited or diffusion-limited process, while for the PS II core and larger complexes the transfer of the excitation energy to the PS II RC may be rate limiting.

Supramolecular organization of photosystem II and its light-harvesting antenna in partially solubilized photosystem II membranes

We present an extended analysis of the organization of green plant photosystem II and its associated light-harvesting antenna using electron microscopy and image analysis. The analysis is based on a large dataset of 16 600 projections of negatively stained PSII-LHCII supercomplexes and megacomplexes prepared by means of three different pretreatments. In addition to our previous work on this system [Boekema, E.J., van Roon, H., Calkoen, F., Bassi, R. and Dekker, J.P. (1999) Biochemistry 38, 2233-2239], the following results were obtained. The rotational orientation of trimeric LHCII at the S, M and L binding positions was determined. It was found that compared to the S trimer, the M and L trimers are rotationally shifted by about -20 degrees and -50 degrees, respectively. The number of projections with empty CP29, CP26 and CP24 binding sites was found to be about 0, 18 and 4%, respectively. We suggest that CP26 and CP24 are not required for the binding of trimeric LHCII at any of the three binding positions. A new type of megacomplex was observed with a characteristic windmill-like shape. This type III megacomplex consists of two C2S2 supercomplexes connected at their CP26 tips. Structural variation in the region of the central dimeric photosystem II complex was found to occur at one specific position near the periphery of the complex. We attribute this variation to the partial absence of an extrinsic polypeptide or one or more small intrinsic membrane proteins.

Spectroscopic properties of the CP43 core antenna protein of photosystem II

CP43 is a chlorophyll-protein complex that funnels excitation energy from the main light-harvesting system of photosystem II to the photochemical reaction center. We purified CP43 from spinach photosystem II membranes in the presence of the nonionic detergent n-dodecyl-beta,D-maltoside and recorded its spectroscopic properties at various temperatures between 4 and 293 K by a number of polarized absorption and fluorescence techniques, fluorescence line narrowing, and Stark spectroscopy. The results indicate two "red" states in the Q(y) absorption region of the chlorophylls. The first peaks at 682.5 nm at 4 K, has an extremely narrow bandwidth with a full width at half-maximum of approximately 2.7 nm (58 cm(-1)) at 4 K, and has the oscillator strength of a single chlorophyll. The second peaks at approximately 679 nm, has a much broader bandshape, is caused by several excitonically interacting chlorophylls, and is responsible for all 4 K absorption at wavelengths longer than 685 nm. The Stark spectrum of CP43 resembles the first derivative of the absorption spectrum and has an exceptionally small overall size, which we attribute to opposing orientations of the monomer dipole moments of the excitonically coupled pigments.

Heptameric association of light-harvesting complex II trimers in partially solubilized photosystem II membranes

We report a structural characterization by electron microscopy and image analysis of a supramolecular complex consisting of seven trimeric light-harvesting complex II proteins. The complex was readily observed in partially-solubilized Tris-washed photosystem II membranes from spinach but was also found to occur, with a low frequency, in oxygen-evolving photosystem II membranes. The structure reveals six peripheral trimers with the same rotational orientation and a central trimer with the opposite orientation. We conclude that the heptamer represents a naturally occurring aggregation state of part of the light-harvesting complex II trimers in the thylakoid membranes.

Multiple types of association of photosystem II and its light-harvesting antenna in partially solubilized photosystem II membranes

Photosystem II is a multisubunit pigment-protein complex embedded in the thylakoid membranes of chloroplasts. It utilizes light for photochemical energy conversion, and is heavily involved in the regulation of the energy flow. We investigated the structural organization of photosystem II and its associated light-harvesting antenna by electron microscopy, multivariate statistical analysis, and classification procedures on partially solubilized photosystem II membranes from spinach. Observation by electron microscopy shortly after a mild disruption of freshly prepared membranes with the detergent n-dodecyl-alpha,D-maltoside revealed the presence of several large supramolecular complexes. In addition to the previously reported supercomplexes [Boekema, E. J., van Roon, H., and Dekker, J. P. (1998) FEBS Lett. 424, 95-99], we observed complexes with the major trimeric chlorophyll a/b protein (LHCII) in a third, L-type of binding position (C2S2M0-2L1-2), and two different types of megacomplexes, both identified as dimeric associations of supercomplexes with LHCII in two types of binding sites (C4S4M2-4). We conclude that the association of photosystem II and its associated light-harvesting antenna is intrinsically heterogeneous, and that the minor CP26 and CP24 proteins play a crucial role in the supramolecular organization of the complete photosystem. We suggest that different types of organization form the structural basis for photosystem II to specifically react to changing light and stress conditions, by providing different routes of excitation energy transfer.

Fluorescence and absorption spectroscopy of the weakly fluorescent chlorophyll a in cytochrome b6f of Synechocystis PCC6803

A spectroscopic characterization of the chlorophyll a (Chl) molecule in the monomeric cytochrome b6f complex (Cytb6f) isolated from the cyanobacterium Synechocystis PCC6803 is presented. The fluorescence lifetime and quantum yield have been determined, and it is shown that Chl in Cytb6f has an excited-state lifetime that is 20 times smaller than that of Chl in methanol. This shortening of the Chl excited state lifetime is not caused by an increased rate of intersystem crossing. Most probably it is due to quenching by a nearby amino acid. It is suggested that this quenching is a mechanism for preventing the formation of Chl triplets, which can lead to the formation of harmful singlet oxygen. Using site-selected fluorescence spectroscopy, detailed information on vibrational frequencies in both the ground and Qy excited states has been obtained. The vibrational frequencies indicate that the Chl molecule has one axial ligand bound to its central magnesium and accepts a hydrogen bond to its 13(1)-keto carbonyl. The results show that the Chl binds to a well-defined pocket of the protein and experiences several close contacts with nearby amino acids. From the site-selected fluorescence spectra, it is further concluded that the electron-phonon coupling is moderately strong. Simulations of both the site-selected fluorescence spectra and the temperature dependence of absorption and fluorescence spectra are presented. These simulations indicate that the Huang-Rhys factor characterizing the electron-phonon coupling strength is between 0.6 and 0.9. The width of the Gaussian inhomogeneous distribution function is 210 +/- 10 cm-1.

The nature of the excited state of the reaction center of photosystem II of green plants: a high-resolution fluorescence spectroscopy study

We studied the electronically excited state of the isolated reaction center of photosystem II with high-resolution fluorescence spectroscopy at 5 K and compared the obtained spectral features with those obtained earlier for the primary electron donor. The results show that there is a striking resemblance between the emitting and charge-separating states in the photosystem II reaction center, such as a very similar shape of the phonon wing with characteristic features at 19 and 80 cm-1, almost identical frequencies of a number of vibrational modes, a very similar double-Gaussian shape of the inhomogeneous distribution function, and relatively strong electron-phonon coupling for both states. We suggest that the emission at 5 K originates either from an exciton state delocalized over the inactive branch of the photosystem or from a fraction of the primary electron donor that is long-lived at 5 K. The latter possibility can be explained by a distribution of the free energy difference of the primary charge separation reaction around zero. Both possibilities are in line with the idea that the state that drives primary charge separation in the reaction center of photosystem II is a collective state, with contributions from all chlorophyll molecules in the central part of the complex.

Energy transfer and charge separation in photosystem I: P700 oxidation upon selective excitation of the long-wavelength antenna chlorophylls of Synechococcus elongatus

Photosystem I of the cyanobacterium Synechococcus elongatus contains two spectral pools of chlorophylls called C-708 and C-719 that absorb at longer wavelengths than the primary electron donor P700. We investigated the relative quantum yields of photochemical charge separation and fluorescence as a function of excitation wavelength and temperature in trimeric and monomeric photosystem I complexes of this cyanobacterium. The monomeric complexes are characterized by a reduced content of the C-719 spectral form. At room temperature, an analysis of the wavelength dependence of P700 oxidation indicated that all absorbed light, even of wavelengths of up to 750 nm, has the same probability of resulting in a stable P700 photooxidation. Upon cooling from 295 K to 5 K, the nonselectively excited steady-state emission increased by 11- and 16-fold in the trimeric and monomeric complexes, respectively, whereas the quantum yield of P700 oxidation decreased 2.2- and 1.7-fold. Fluorescence excitation spectra at 5 K indicate that the fluorescence quantum yield further increases upon scanning of the excitation wavelength from 690 nm to 710 nm, whereas the quantum yield of P700 oxidation decreases significantly upon excitation at wavelengths longer than 700 nm. Based on these findings, we conclude that at 5 K the excited state is not equilibrated over the antenna before charge separation occurs, and that approximately 50% of the excitations reach P700 before they become irreversibly trapped on one of the long-wavelength antenna pigments. Possible spatial organizations of the long-wavelength antenna pigments in the three-dimensional structure of photosystem I are discussed.

Specific association of photosystem II and light-harvesting complex II in partially solubilized photosystem II membranes

In this study, we report the structural characterization of photosystem II complexes obtained from partially solubilized photosystem II membranes. Direct observation by electron microscopy, within a few minutes after a mild disruption of the membranes with the detergent n-dodecyl-alpha,D-maltoside, revealed the presence of several large supramolecular complexes. Images of these complexes were subjected to multivariate statistical analysis and classification procedures, resolving a new complex consisting of the previously characterized dimeric supercomplex of photosystem II and light-harvesting complex II [Boekema et al., Proc. Natl. Acad. Sci. USA 92 (1995) 175-179] and two additional, symmetrically organized protein masses each containing a second type of trimeric light-harvesting II complex. We conclude that large and labile integral membrane proteins, such as photosystem II, can be quickly structurally characterized without extensive purification.

Role of Arg180 of the D2 protein in photosystem II structure and function

On the basis of sequence comparison with the M subunit of the reaction center of purple bacteria, no residues in photosystem II can be clearly identified that may be predicted to correspond to the His residue that binds one of the accessory bacteriochlorophylls in the purple bacterial reaction center. However, the Arg180 residue of the D2 protein is close to where this residue is predicted to be and could conceivably serve as a chlorophyll ligand. To analyze the function of Arg180, it was changed to nine different amino acids in the cyanobacterium Synechocystis sp. PCC 6803. Except for the Arg180-->Gln (R180Q) mutant, the resulting strains were no longer photoautotrophic. The properties of photosystem II upon mutation of Arg180 were probed in strains from which photosystem I had been deleted genetically. Mutations at the Arg180 residue affected oxygen evolution capacity and the amount of photosystem II that was present in thylakoids. Surprisingly, in the Arg180 mutants, EPR signals that may originate from the oxidized redoxactive Tyr160 of the D2 protein (Y(D)ox) were small and generally did not resemble the usual signal IIs, signifying an effect of the Arg180 mutations on the environment surrounding Tyr160. In addition, in most mutants, the charge recombination kinetics between the primary electron-accepting quinone in photosystem II (Q(A)-) and oxidized species on the donor side were faster upon introducing mutations at Arg180 suggesting an increased steady-state concentration of P680+ in the mutants. However, Arg180 mutations also affected Q(A)- oxidation by the secondary electron-accepting quinone (Q(B)). HPLC analysis showed that, in the Arg180 mutants that were assayed, the pheophytin/chlorophyll ratio of photosystem II had not changed, indicating that the mutations did not lead to a pheophytinization of one of the chlorophyll molecules. Even though the results presented do not provide positive evidence that Arg180 of the D2 protein corresponds in function to the ligand to the central Mg in an accessory bacteriochlorophyll in reaction centers of purple bacteria, it is clear that changes in Arg180 greatly affect Tyr160 and P680. Various scenarios are discussed that are compatible with the data presented, and include an apparently close interaction between Arg180, His189, and Tyr160, and the possibility of the involvement of multiple chlorophylls to together form P680.

Charge separation in the reaction center of photosystem II studied as a function of temperature

In photosystem II of green plants the key photosynthetic reaction consists of the transfer of an electron from the primary donor called P680 to a nearby pheophytin molecule. We analyzed the temperature dependence of this reaction by subpicosecond transient absorption spectroscopy over the temperature range 20-240 K using isolated photosystem II reaction centers from spinach. After excitation in the red edge of the Qy absorption band, the decay of the excited state can conveniently be described by two kinetic components that both accelerate with temperature. This temperature behavior differs remarkably from that observed in purple bacterial reaction centers. We attribute the first component, which accelerates from 2.6 ps at 20 K to 0.4 ps at 240 K, to charge separation after direct excitation of P680, and explain its temperature dependence by an intermediate that lies in energy above the singlet-excited P680 and that possibly has charge-transfer character. The second component accelerates from 120 ps at 20 K to 18 ps at 240 K and is attributed to charge separation after direct excitation of the "trap" state near-degenerate with P680 and subsequent slow energy transfer from this trap state to P680. We suggest that the slow energy transfer from the trap state to P680 plays an important role in the kinetics of radical pair formation at room temperature.

Purification and spectroscopic characterization of photosystem II reaction center complexes isolated with or without Triton X-100

The pigment composition of the isolated photosystem II reaction center complex in its most stable and pure form currently is a matter of considerable debate. In this contribution, we present a new method based on a combination of gel filtration chromatography and diode array detection to analyze the composition of photosystem II reaction center preparations. We show that the method is very sensitive for the detection of contaminants such as the core antenna protein CP47, pigment-free and denatured reaction center proteins, and unbound chlorophyll and pheophytin molecules. We also present a method by which the photosystem II reaction center complex is highly purified without using Triton X-100, and we show that in this preparation the contamination with CP47 is less than 0.1%. The results strongly indicate that the photosystem II reaction center complex in its most stable and pure form binds six chlorophyll a, two pheophytin a, and two beta-carotene molecules and that the main effect of Triton X-100 is the extraction of beta-carotene from the complex. Analysis of 4 K absorption and emission spectra indicates that the spectroscopic properties of this preparation are similar to those obtained by a short Triton X-100 treatment. In contrast, preparations obtained by long Triton X-100 treatment show decreased absorption of the shoulder at 684 nm in the 4 K absorption spectrum and an increased number of pigments that trap excitation energy at very low temperatures. We conclude that the 684 nm shoulder in the 4 K absorption spectrum should at least in part be attributed to the primary electron donor of photosystem II.

Polarized site-selective fluorescence spectroscopy of the long-wavelength emitting chlorophylls in isolated Photosystem I particles of Synechococcus elongatus

Isolated trimeric Photosystem I complexes of the cyanobacterium Synechococcus elongatus have been studied with absorption spectroscopy and site-selective polarized fluorescence spectroscopy at cryogenic temperatures. The 4 K absorption spectrum exhibits a clear and distinct peak at 710 nm and shoulders near 720, 698 and 692 nm apart from the strong absorption profile located at 680 nm. Deconvoluting the 4 K absorption spectrum with Gaussian components revealed that Synechococcus elongatus contains two types of long-wavelength pigments peaking at 708 nm and 719 nm, which we denoted C-708 and C-719, respectively. An estimate of the oscillator strengths revealed that Synechococcus elongatus contains about 4-5 C-708 pigments and 5-6 C-719 pigments. At 4 K and for excitation wavelengths shorter than 712 nm, the emission maximum appeared at 731 nm. For excitation wavelengths longer than 712 nm, the emission maximum shifted to the red, and for excitation in the far red edge of the absorption spectrum the emission maximum was observed 10-11 nm to the red with respect to the excitation wavelength, which indicates that the Stokes shift of C-719 is 10-11 nm. The fluorescence anisotropy, as calculated in the emission maximum, reached a maximal anisotropy of r=0.35 for excitation in the far red edge of the absorption spectrum (at and above 730 nm), and showed a complicated behavior for excitation at shorter wavelengths. The results suggest efficient energy transfer routes between C-708 and C-719 pigments and also among the C-719 pigments.

A multimer model for P680, the primary electron donor of photosystem II

We consider a model of the photosystem II (PS II) reaction center in which its spectral properties result from weak (approximately 100 cm-1) excitonic interactions between the majority of reaction center chlorins. Such a model is consistent with a structure similar to that of the reaction center of purple bacteria but with a reduced coupling of the chlorophyll special pair. We find that this model is consistent with many experimental studies of PS II. The similarity in magnitude of the exciton coupling and energetic disorder in PS II results in the exciton states being structurally highly heterogeneous. This model suggests that P680, the primary electron donor of PS II, should not be considered a dimer but a multimer of several weakly coupled pigments, including the pheophytin electron acceptor. We thus conclude that even if the reaction center of PS II is structurally similar to that of purple bacteria, its spectroscopy and primary photochemistry may be very different.

Description of energy migration and trapping in photosystem I by a model with two distance scaling parameters

The energy transfer and trapping kinetics in the core antenna of Photosystem I are described in a new model in which the distance between the core antenna chlorophylls and P700 is proposed to be considerably longer than the distance between the chlorophylls within the antenna. Structurally, the model describes the Photosystem I core antenna as a regular sphere around P700, while energetically it consists of three levels representing the bulk antenna, P700 and the red-shifted antenna pigments absorbing at longer wavelength than P700, respectively. It is shown that the model explains experimental results obtained from the Photosystem I complex of the cyanobacterium Synechococcus sp. (A.R. Holzwarth, G. Schatz, H Brock, and E. Bittersman (1993) Biophys. J. 64: 1813-1826) quite well, and that no unrealistic charge separation rate and organization of the long-wavelength pigments has to be assumed. We suggest that excitation energy transfer and trapping in Photosystem I should be described as a 'transfer-to-the-trap'-limited process.

Triplet and fluorescing states of the CP47 antenna complex of photosystem II studied as a function of temperature

Fluorescence emission and triplet-minus-singlet (T-S) absorption difference spectra of the CP47 core antenna complex of photosystem II were measured as a function of temperature and compared to those of chlorophyll a in Triton X-100. Two spectral species were found in the chlorophyll T-S spectra of CP47, which may arise from a difference in ligation of the pigments or from an additional hydrogen bond, similar to what has been found for Chl molecules in a variety of solvents. The T-S spectra show that the lowest lying state in CP47 is at approximately 685 nm and gives rise to fluorescence at 690 nm at 4 K. The fluorescence quantum yield is 0.11 +/- 0.03 at 4 K, the chlorophyll triplet yield is 0.16 +/- 0.03. Carotenoid triplets are formed efficiently at 4 K through triplet transfer from chlorophyll with a yield of 0.15 +/- 0.02. The major decay channel of the lowest excited state in CP47 is internal conversion, with a quantum yield of about 0.58. Increase of the temperature results in a broadening and blue shift of the spectra due to the equilibration of the excitation over the antenna pigments. Upon increasing the temperature, a decrease of the fluorescence and triplet yields is observed to, at 270 K, a value of about 55% of the low temperature value. This decrease is significantly larger than of chlorophyll a in Triton X-100. Although the coupling to low-frequency phonon or vibration modes of the pigments is probably intermediate in CP47, the temperature dependence of the triplet and fluorescence quantum yield can be modeled using the energy gap law in the strong coupling limit of Englman and Jortner (1970. J. Mol. Phys. 18:145-164) for non-radiative decays. This yields for CP47 an average frequency of the promoting/accepting modes of 350 cm-1 with an activation energy of 650 cm-1 for internal conversion and activationless intersystem crossing to the triplet state through a promoting mode with a frequency of 180 cm-1. For chlorophyll a in Triton X-100 the average frequency of the promoting modes for non-radiative decay is very similar, but the activation energy (300 cm-1) is significantly smaller.

Spectroscopic characterization of three different monomeric forms of the main chlorophyll a/b binding protein from chloroplast membranes

A detailed comparison has been made between dichroic steady-state spectroscopic properties at 77 K of several trimeric and monomeric forms of the major chlorophyll a/b binding protein (LHC-II) from pea. Monomeric forms were obtained by applying high concentrations of nonionic detergents, by a lipase treatment, or by a chymotrypsin/trypsin treatment. The latter treatments removed phosphatidyl glycerol essential for trimer formation. The absorption and dichroism spectra indicate that for trimeric LHC-II the chlorophyll b absorption region is centered around 649 nm and is composed of at least five subbands near 640, 647, 649, 652, and 656 nm. The chlorophyll a absorption region is centered around 670 nm and is composed of at least five bands near 661, 668, 671, 673, and 676 nm. The chlorophyll b band near 647 and 652 nm and the chlorophyll a bands near 668 and 673 nm are absent in the circular dichroism spectrum after monomerization. A configuration in which pigments of the same nature located on different monomers become excitonically coupled in the trimer could explain these results. In monomers obtained in high concentrations of nonionic detergents, no additional bands have disappeared, but the absorption spectra of the other two types of monomers lack the bands at 640 and 661 nm. These monomers have lost some chlorophyll a and b according to the fluorescence emission spectra, which show contributions from free chlorophyll a and b. The results suggest that phosphatidyl glycerol not only is involved in trimer formation but also has a structural role within the monomers.

Simultaneous measurement of electric birefringence and dichroism. A study on photosystem 1 particles

We have developed a straightforward method to separate linear-dichroism and birefringence contributions to electric-field induced signals in a conventional birefringence setup. The method requires the measurement of electric birefringence for three different angular positions of the analyzer. It is demonstrated that the presence of linear dichroism can significantly influence the measured signals and lead to completely erroneous calculations of the birefringence signal and field-free decay times if its contribution is not taken into account. The new method is used to determine electric birefringence and linear dichroism of trimeric Photosystem 1 complexes from the cyanobacterium Synechocystis PCC 6803 in the detergents n-dodecyl-beta-D-maltoside and n-octyl-beta-D-glucoside. It is concluded that the orientation of the particles in the field is predominantly caused by a permanent electric dipole moment that is directed parallel to the symmetry axis of the particles. Comparison of the decay times obtained with dodecylmaltoside and octylglucoside supports a model in which the thickness of the disc-like complexes remains similar (7-8 nm) upon replacing dodecylmaltoside by octylglucoside, whereas the diameter increases from 14.4 +/- 0.2 to 16.6 +/- 0.2 nm because of an increased thickness of the detergent layer. This change in diameter is in good agreement with electron-microscopy results on Photosystem 2 complexes in dodecylmaltoside and octylglucoside (Dekker, J. P., E. J. Boekema, H. T. Witt, and M. Rögner. 1988. Biochim. Biophys. Acta 936:307-318). The value of approximately 16.6 nm for the diameter of Photosystem 1 trimers in dodecylmaltoside is in good agreement with recent results obtained from electron microscopy in combination with extensive image analysis (Kruip, J., E. J. Boekema, D. Bald, A. F. Boonstra, and M. Rögner. 1993. J. Biol. Chem. 268:23353-23360).

Temperature-dependent triplet and fluorescence quantum yields of the photosystem II reaction center described in a thermodynamic model

A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem II reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 +/- 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem II reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 +/- 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05-0.06 at 60-70 K, increases upon raising the temperature to 0.07 at approximately 165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem II reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+l- is temperature independent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data.

Electron microscopic structural analysis of Photosystem I, Photosystem II, and the cytochrome b6/f complex from green plants and cyanobacteria

Electron microscopy (EM) in combination with image analysis is a powerful technique to study protein structure at low- and high resolution. Since electron micrographs of biological objects are very noisy, substantial improvement of image quality can be obtained by averaging individual projections. Crystallographic and noncrystallographic averaging methods are available and have been applied to study projections of the large protein complexes embedded in photosynthetic membranes from cyanobacteria and higher plants. Results of EM on monomeric and trimeric Photosystem I complexes, on monomeric and dimeric Photosystem II complexes, and on the monomeric cytochrome b6/f complex are discussed.

Flash-induced redox changes in oxygen-evolving spinach Photosystem II core particles

Flash-induced redox reactions in spinach PS II core particles were investigated with absorbance difference spectroscopy in the UV-region and EPR spectroscopy. In the absence of artificial electron acceptors, electron transport was limited to a single turnover. Addition of the electron acceptors DCBQ and ferricyanide restored the characteristic period-four oscillation in the UV absorbance associated with the S-state cycle, but not the period-two oscillation indicative of the alternating appearance and disappearance of a semiquinone at the QB-site. In contrast to PS II membranes, all active centers were in state S1 after dark adaptation. The absorbance increase associated with the S-state transitions on the first two flashes, attributed to the Z(+)S1→ZS2 and Z(+)S2→ZS3 transitions, respectively, had half-times of 95 and 380 μs, similar to those reported for PS II membrane fragments. The decrease due to the Z(+)S3→ZS0 transition on the third flash had a half-time of 4.5 ms, as in salt-washed PS II membrane fragments. On the fourth flash a small, unresolved, increase of less than 3 μs was observed, which might be due to the Z(+)S0→ZS1 transition. The deactivation of the higher S-states was unusually fast and occurred within a few seconds and so was the oxidation of S0 to S1 in the dark, which had a half-time of 2-3 min. The same lifetime was found for tyrosine D(+), which appeared to be formed within milliseconds after the first flash in about 10% inactive centers and after the third and later flashes by active centers in Z(+)S3.

Steady-state polarized light spectroscopy of isolated Photosystem I complexes

Monomeric and trimeric Photosystem I core complexes from the cyanobacterium Synechocystis PCC 6803 and LHC-I containing Photosystem I (PS I-200) complexes from spinach have been characterized by steady-state, polarized light spectroscopy at 77 K. The absorption spectra of the monomeric and trimeric core complexes from Synechocystis were remarkably similar, except for the amplitude of a spectral component at long wavelength, which was about twice as large in the trimeric complexes. This spectral component did not contribute significantly to the CD-spectrum. The (77 K) steady-state emission spectra showed prominent peaks at 724 nm (for the Synechocystis core complexes) and at 735 nm (for PS I-200). A comparison of the excitation spectra of the main emission band and the absorption spectra suggested that a significant part of the excitations do not pass the red pigments before being trapped by P-700. Polarized fluorescence excitation spectra of the monomeric and trimeric core complexes revealed a remarkably high anisotropy (∼0.3) above 705 nm. This suggested one or more of the following possibilities: 1) there is one red-most pigment to which all excitations are directed, 2) there are more red-most pigments but with (almost) parallel orientations, 3) there are more red-most pigments, but they are not connected by energy transfer. The high anisotropy above 705 nm of the trimeric complexes indicated that the long-wavelength pigments on different monomers are not connected by energy transfer. In contrary to the Synechocystis core complexes, the anisotropy spectrum of the LHC I containing complexes from spinach was not constant in the region of the long-wavelength pigments, and decreased significantly below 720 nm, the wavelength where the long-wavelength pigments on the core complexes start to absorb. These results suggested that in spinach the long-wavelength pigments on core and LHC-I are connected by energy transfer and have a non-parallel average Qy(0-0) transitions.

2,4,5-trichlorophenoxyacetic acid influence on 2,6-dinitrotoluene-induced urine genotoxicity in Fischer 344 rats: effect on gastrointestinal microflora and enzyme activity

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) and 2,6-dinitrotoluene (2,6-DNT) are hazardous chemicals that have potential harmful effects. 2,6-DNT is recognized as a hepatotoxicant while 2,4,5-T, a component of Agent Orange, is also suspect. 2,6-DNT requires both oxidative and reductive metabolism to elicit genotoxic effects. To determine what effect 2,4,5-T had on 2,6-DNT metabolism, intestinal enzymes, microbial populations, and urine mutagenicity were examined during 2,4,5-T treatment. Weanling Fischer 344 male rats were treated daily with 54.4 mg/kg 2,4,5-T by gavage for 4 weeks. One, two, and four weeks after the initial 2,4,5-T dose, rats were administered (po) 2,6-DNT (75 mg/kg) and urine was collected for 24 hr in metabolism cages. Azo reductase, nitroreductase, beta-glucuronidase, dechlorinase, and dehydrochlorinase activities were examined concurrently. Treatment of rats for 1 week reduced the transformation of 2,6-DNT to mutagenic urinary metabolites. This was accompanied by a decrease in the fecal anaerobic microorganisms. The elimination of Lactobacillus fermentum from the small intestine and cecum of treated animals accompanied a significant increase in oxygen-tolerant lactobacilli and other unidentified aerobic microorganisms. However, there were no significant alterations in the intestinal enzyme activities examined. By 2 weeks of 2,4,5-T treatment, microbiota and urine genotoxicity returned to the levels observed in control animals. This trend continued for the duration of the experiment. After 2 weeks, while cecal nitroreductase and azo reductase activities increased, small intestinal beta-glucuronidase activity decreased. By 4 weeks, treated and untreated animal intestinal enzyme activities were indistinguishable.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid and simple isolation of pure photosystem II core and reaction center particles from spinach

Pure and active oxygen-evolving PS II core particles containing 35 Chl per reaction center were isolated with 75% yield from spinach PS II membrane fragments by incubation with n-dodecyl-β-D-maltoside and a rapid one step anion-exchange separation. By Triton X-100 treatment on the column these particles could be converted with 55% yield to pure and active PS II reaction center particles, which contained 6 Chl per reaction center.

Comparative gastrointestinal enzyme activity and activation of the promutagen 2,6-dinitrotoluene in male CD-1 mice and male Fischer 344 rats

Comparative intestinal nitroreductase, azo reductase, beta-glucuronidase, dechlorinase and dehydrochlorinase activities in young male Fischer 344 rats and young male CD-1 mice were measured in vitro while the comparative biotransformation of 2,6-dinitrotoluene to mutagenic metabolites was determined in vivo. The mice, which exhibit a high spontaneous incidence of hepatomas, had markedly greater nitroreductase activity and metabolized significantly more 2,6-dinitrotoluene to mutagenic metabolites than did Fischer 344 rats, which show a low incidence of liver tumors. Results of this study indicate that species differences in the incidence of hepatomas may be influenced by microbial flora and/or the biotransformation of xenobiotics in the G.I. tract.

Characterization by electron microscopy of isolated particles and two-dimensional crystals of the CP47-D1-D2-cytochrome b-559 complex of photosystem II

A photosystem II complex containing the reaction center proteins D1 and D2, a 47-kDa chlorophyll-binding protein (CP47), and cytochrome b-559 was isolated with high yield, purity, and homogeneity; small but well-ordered two-dimensional crystals were prepared from the particles. The crystals and the isolated particles were analyzed by electron microscopy using negatively stained specimens. The information of 20 different digitized crystals was combined by alignment programs based on correlation methods to obtain a final average. The calculated diffraction pattern, with spots up to a resolution of 2.5 nm, and the optical diffraction pattern of a single crystal indicate that the plane group is p22121 (also called p2gg) and that the unit cell is rectangular with parameters of 23.5 x 16.0 nm, containing four stain-excluding monomers (two face-up and two face-down). In projection, the monomers have an asymmetrical shape with a length of 10 nm, a maximal width of 7.5 nm, and a height of 6 nm; their molecular mass is 175 +/- 40 kDa.