Seven new genes designated dsrLJOPNSR were identified immediately downstream of dsrABEFHCMK, completing the dsr gene cluster of the phototrophic sulfur bacterium Allochromatium vinosum D (DSM 180 T ). Interposon mutagenesis proved an essential role of the encoded proteins for the oxidation of intracellular sulfur, an obligate intermediate during the oxidation of sulfide and thiosulfate. While dsrR and dsrS encode cytoplasmic proteins of unknown function, the other genes encode a predicted NADPH:acceptor oxidoreductase (DsrL), a triheme c-type cytochrome (DsrJ), a periplasmic iron-sulfur protein (DsrO), and an integral membrane protein (DsrP). DsrN resembles cobyrinic acid a,c-diamide synthases and is probably involved in the biosynthesis of siro(heme)amide, the prosthetic group of the dsrAB-encoded sulfite reductase. The presence of most predicted Dsr proteins in A. vinosum was verified by Western blot analysis. With the exception of the constitutively present DsrC, the formation of Dsr gene products was greatly enhanced by sulfide. DsrEFH were purified from the soluble fraction and constitute a soluble ␣ 2 ␤ 2 ␥ 2 -structured 75-kDa holoprotein. DsrKJO were purified from membranes pointing at the presence of a transmembrane electron-transporting complex consisting of DsrKMJOP. In accordance with the suggestion that related complexes from dissimilatory sulfate reducers transfer electrons to sulfite reductase, the A. vinosum Dsr complex is copurified with sulfite reductase, DsrEFH, and DsrC. We therefore now have an ideal and unique possibility to study the interaction of sulfite reductase with other proteins and to clarify the long-standing problem of electron transport from and to sulfite reductase, not only in phototrophic bacteria but also in sulfate-reducing prokaryotes.Phototrophic purple and green sulfur-oxidizing bacteria use sulfur compounds as electron donors for reductive carbon dioxide fixation during photolithotrophic growth (7, 10). In these organisms, light energy is used to transfer electrons from sulfur compounds to the level of the more highly reducing electron carriers NAD(P) ϩ and ferredoxin. In our laboratory we seek to understand oxidative sulfur metabolism in anoxygenic phototrophic bacteria by using the genetically accessible ␥-proteobacterium Allochromatium vinosum (formerly Chromatium vinosum [34]) as our model organism. It is a purple sulfur bacterium belonging to the family Chromatiaceae. A. vinosum carries out the complete eight-electron oxidations of sulfide and thiosulfate to sulfate. Intracellularly stored sulfur globules are an obligate intermediate in the process (57). The sulfur in the globules is present in the molecular structure of sulfur chains (58) and is enclosed by a protein envelope, a feature shared by most if not all of the chemotrophic sulfur-oxidizing bacteria that form intracellular sulfur globules (9, 14, 51). Topologically, the sulfur globules of A. vinosum and probably of other members of the Chromatiaceae are located extracytoplasmically, in the periplasm (51).O...

We have undertaken an integrated chemical and morphological comparison of the amyloid-␤ (A␤) molecules and the amyloid plaques present in the brains of APP23 transgenic (tg) mice and human Alzheimer's disease (AD) patients. Despite an apparent overall structural resemblance to AD pathology, our detailed chemical analyses revealed that although the amyloid plaques characteristic of AD contain cores that are highly resistant to chemical and physical disruption, the tg mice produced amyloid cores that were completely soluble in buffers containing SDS. A␤ chemical alterations account for the extreme stability of AD plaque core amyloid. The corresponding lack of post-translational modifications such as N-terminal degradation, isomerization, racemization, pyroglutamyl formation, oxidation, and covalently linked dimers in tg mouse A␤ provides an explanation for the differences in solubility between human AD and the APP23 tg mouse plaques. We hypothesize either that insufficient time is available for A␤ structural modifications or that the complex species-specific environment of the human disease is not precisely replicated in the tg mice. The appraisal of therapeutic agents or protocols in these animal models must be judged in the context of the lack of complete equivalence between the transgenic mouse plaques and the human AD lesions. Alzheimer's disease (AD)1 is a progressive neurodegenerative disorder characterized by the presence of extracellular amyloid plaques composed principally of amyloid-␤ (A␤) surrounded by dystrophic neurites (1). This association and the realization that the basis of certain early-onset familial forms of AD seems to be the enhanced production of one or more A␤ peptides have led to the hypothesis that A␤ is intimately involved in the AD pathogenic process (2). A promising experimental approach to unraveling the role(s) of A␤ in AD pathology has been the construction and characterization of transgenic mice that overexpress the amyloid precursor protein (APP) (3-12). Several transgenic mouse lines have been described that produce A␤ deposits that accumulate in an agedependent fashion and morphologically resemble the senile plaques characteristic of human AD (3,6,8,12,37,38).The APP23 transgenic (tg) mice contain an APP751 cDNA with the Swedish familial AD mutation under the control of the neuron-specific Thy-1 promoter and express this human gene at levels 7-fold greater than endogenous murine APP (12). Longitudinal studies of these mice have revealed that extracellular amyloid deposits become evident as the APP23 tg mice age. These deposits exhibit, at their earliest appearance, the Congo red birefringence characteristic of the dense core plaques of human AD (12). A gradual progression from a diffuse deposit to a dense plaque is not a feature of the APP23 tg mouse pathology, paralleling our previous finding (13) that the diffuse amyloid deposits of AD do not represent a precursor developmental stage of senile plaques.A transgenic mouse model system that faithfully mimics every aspect of AD h...

Experiments with amyloid-␤ (A␤)-42-immunized transgenic mouse models of Alzheimer's disease have revealed amyloid plaque disruption and apparent cognitive function recovery. Neuropathological examination of patients vaccinated against purified A␤-42 (AN-1792) has demonstrated that senile plaque disruption occurred in immunized humans as well. Here, we examined tissue histology and quantified and biochemically characterized the remnant amyloid peptides in the gray and white matter and leptomeningeal/cortical vessels of two AN-1792-vaccinated patients, one of whom developed meningoencephalitis. Compact core and diffuse amyloid deposits in both vaccinated individuals were focally absent in some regions. Although parenchymal amyloid was focally disaggregated, vascular deposits were relatively preserved or even increased. Immunoassay revealed that total soluble amyloid levels were sharply elevated in vaccinated patient gray and white matter compared with Alzheimer's disease cases. Our experiments suggest that although immunization disrupted amyloid deposits, vascular capture prevented largescale egress of A␤ peptides. Trapped, solubilized amyloid peptides may ultimately have cascading toxic effects on cerebrovascular, gray and white matter tissues. Anti-amyloid immunization may be most effective not as therapeutic or mitigating measures but as a prophylactic measure when A␤ deposition is still minimal. This may allow A␤ mobilization under conditions in which drainage and degradation of these toxic peptides is efficient.

SummaryTwo different pathways for thiosulphate oxidation are present in the purple sulphur bacterium Allochromatium vinosum: oxidation to tetrathionate and complete oxidation to sulphate with obligatory formation of sulphur globules as intermediates. The tetrathionate:sulphate ratio is strongly pH-dependent with tetrathionate formation being preferred under acidic conditions. Thiosulphate dehydrogenase, a constitutively expressed monomeric 30 kDa c-type cytochrome with a pH optimum at pH 4.2 catalyses tetrathionate formation. A periplasmic thiosulphateoxidizing multienzyme complex (Sox) has been described to be responsible for formation of sulphate from thiosulphate in chemotrophic and phototrophic sulphur oxidizers that do not form sulphur deposits. In the sulphur-storing A. vinosum we identified five sox genes in two independent loci (soxBXA and soxYZ). For SoxA a thiosulphate-dependent induction of expression, above a low constitutive level, was observed. Three sox-encoded proteins were purified: the heterodimeric c-type cytochrome SoxXA, the monomeric SoxB and the heterodimeric SoxYZ. Gene inactivation and complementation experiments proved these proteins to be indispensable for thiosulphate oxidation to sulphate. The intermediary formation of sulphur globules in A. vinosum appears to be related to the lack of soxCD genes, the products of which are proposed to oxidize SoxY-bound sulphane sulphur. In their absence the latter is instead transferred to growing sulphur globules.

Bacteriopheophytin, the magesium-free base of bacteriochlorophyll, undergoes reversible one-electron reduction in organic solvents to yield an anionic free radical with characteristic optical and electron spin resonance spectra. The which was pumped away to codistill water, and the pigment was dried under reduced pressure (10-3 Pa) for several hours. RESULTSThe anion radical of bacteriopheophytin BPh exhibits a reversible, one electron, halfwave reduction potential E1/2 of -0.58 against NHE in CH2Cl2.

The gene for a reaction center core polypeptide from the anoxygenic photosynthetic bacterium Heliobacillus mobilis was cloned and sequenced. The deduced amino acid sequence consists of 609 residues with a molecular mass of 68 kDa. An adjacent open reading frame is not transcribed under our experimental conditions. No evidence for a second related reaction center core gene was found. The primary sequence of the reaction center protein (P800 protein) shows a high percentage of sequence identity to photosystem I in a cysteine-containing loop, which is the putative binding site of the iron-sulfur center FX and in the preceding hydrophobic region. Our data imply a homodimeric organization of the reaction center. This is fundamentally different from photosystem I and most other photosynthetic reaction centers, where the reaction center core is composed of two similar but nonidentical subunits.

Many sulfide-oxidizing organisms, including the photosynthetic sulfur bacteria, store sulfur in "sulfur globules" that are readily detected microscopically. The chemical form of sulfur in these globules is currently the focus of a debate, because they have been described as "liquid" by some observers, although no known allotrope of sulfur is liquid at physiological temperatures. In the present work we have used sulfur K-edge X-ray absorption spectroscopy to identify and quantify the chemical forms of sulfur in a variety of bacterial cells, including photosynthetic sulfur bacteria. We have also taken advantage of X-ray fluorescence self-absorption to derive estimates of the size and density of the sulfur globules in photosynthetic bacteria. We find that the form of sulfur that most resembles the globule sulfur is simply solid S(8), rather than more exotic forms previously proposed.

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