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PubMed1978

Quantum yield and image contrast of bacteriochlorophyll monolayers in photoelectron microscopy.

The photoelectron quantum yield spectrum of bacteriochlorophyll aGg (Bchl a ) from Rhodospirillum rubrum was determined in order to evaluate the possibility of mapping photoreceptor distribution and organization in bacterial chromatophores. The quantum yield is on the order of 1 X 10(-3) electrons/incident photon at 180 nm and decreases to 2.5 X 10(-5) electrons/incident photon at 230 nm. Photoelectron micrographs confirm the high contrast predicted between monolayers of Bchl a against a lipid background (calcium arachidate). A significant contrast difference is found between the two monolayer orientations, demonstrating that photoelectron microscopy is a sensitive detector of asymmetry in Bch1 a monolayers.

Bacteriochlorophylls↗

PubMed2003

Essential role for puma in development of postembryonic neural crest-derived cell lineages in zebrafish.

Multipotent neural crest stem cells have been identified in late gestation amniote embryos. Yet, significant questions remain about the mechanisms by which these cells are generated, maintained, and recruited during postembryonic development. The zebrafish, Danio rerio, offers an opportunity to identify genes essential for these processes, by screening for mutants with defects in traits likely to depend on these cells during metamorphosis and adult life. One such trait is the pigment pattern formed by neural crest-derived pigment cells, or chromatophores, which include black melanophores, yellow xanthophores, and iridescent iridophores. Previous analyses have demonstrated that the adult zebrafish pigment pattern depends on the de novo differentiation of latent precursor cells during both early and late phases of pigment pattern metamorphosis. To better understand the development of these cells, in this study, we analyze the zebrafish puma mutant, which ablates most of the adult melanophores that differentiate during metamorphosis, but leaves intact early larval melanophores that differentiate during embryogenesis. We use epistasis analyses to show that puma promotes the development of both early-appearing metamorphic melanophores that depend on the kit receptor tyrosine kinase, as well as late-appearing metamorphic melanophores that depend on both the G-protein-coupled endothelin receptor b1 (ednrb1) and the kit-related fms receptor tyrosine kinase. We further demonstrate that, during pigment pattern metamorphosis, puma mutants have deficiencies in the numbers of cells expressing transcripts for kit, ednrb1, and fms, as well as the HMG domain transcription factor sox10. Because the puma mutant phenotype is temperature-sensitive, we use temperature-shift experiments to identify a critical period for puma activity during pigment pattern metamorphosis. Finally, we use cell transplantations to show that puma acts cell-autonomously to promote the expansion of pigment cell lineages during metamorphosis. These results suggest a model for the lineage diversification of neural crest stem cells during zebrafish postembryonic development.

Animals↗

PubMed1997

A single mutation in the M-subunit of Rhodospirillum rubrum confers herbicide resistance.

Cells of the photosynthetic bacterium Rhodospirillum rubrum were rendered resistant against the inhibitor 2-(1-phenyl)ethylamino-3-propionylamino-4-cyano-thiazole (PPCTH). Electron transport in reaction centers prepared from one of the mutants (M6) was neither inhibited by PPCTH and other NH-thiazoles nor terbutryn. These inhibitors are known to bind at the Q(B) site of the L-subunit. Compared to the wild type, chromatophores from M6 exhibited strongly altered Q(B)- Fe2+ and Q(A)- Fe2+ EPR signals. Inhibitor resistance is due to a mutation in the bacterial reaction center M-subunit, where Glu234 is exchanged against Lys. This is the first example of an inhibitor resistance in the Q(B) site caused by a mutation in the M-subunit.

Drug Resistance, Microbial↗

PubMed1998

The cytochrome bc1 complex of Rhodobacter capsulatus: ubiquinol oxidation in a dimeric Q-cycle?

We studied the cytochrome bc1 complex (hereafter bc) by flash excitation of Rhodobacter capsulatis chromatophores. The reduction of the high-potential heme b(h), of cytochrome b (at 561 nm) and of cytochromes c (at 552 nm) and the electrochromic absorption transients (at 524 nm) were monitored after the first and second flashes of light, respectively. We kept the ubiquinone pool oxidized in the dark and concerned for the ubiquinol formation in the photosynthetic reaction center only after the second flash. Surprisingly, the first flash caused the oxidation of about one ubiquinol per bc dimer. Based on these and other data we propose a dimeric Q-cycle where the energetically unfavorable oxidation of the first ubiquinol molecule by one of the bc monomers is driven by the energetically favorable oxidation of the second ubiquinol by the other bc monomer resulting in a pairwise oxidation of ubiquinol molecules by the dimeric bc in the dark. The residual unpaired ubiquinol supposedly remains on the enzyme and is then oxidized after the first flash.

Electron Transport↗

PubMed1998

Non-inhibiting perturbation of the primary energy conversion site (Qo site) in Rhodobacter capsulatus ubihydroquinone: cytochrome c oxidoreductase (cytochrome bc1 complex).

Ethanol added to Rhodobacter capsulatus chromatophore membranes containing the cytochrome bc1 complex effectively uncouples the sensitivity of the [2Fe-2S] cluster EPR spectrum to the number and redox state of ubiquinone/ubihydroquinone within the Qo site. Ethanol has no effect upon the rate of catalysis, leading to a non-inhibiting perturbation of cytochrome bc1 function. We suggest that displacement occurs by ethanol acting from the aqueous phase to successfully compete with the Qo site ubiquinones and water to hydrogen bond the N(epsilon)H atom(s) of the coordinating [2Fe-2S] cluster histidines.

Binding Sites↗

PubMed1999

ATP-synthase of Rhodobacter capsulatus: coupling of proton flow through F0 to reactions in F1 under the ATP synthesis and slip conditions.

A stepwise increasing membrane potential was generated in chromatophores of the phototrophic bacterium Rhodobacter capsulatus by illumination with short flashes of light. Proton transfer through ATP-synthase (measured by electrochromic carotenoid bandshift and by pH-indicators) and ATP release (measured by luminescence of luciferin-luciferase) were monitored. The ratio between the amount of protons translocated by F0F1 and the ATP yield decreased with the flash number from an apparent value of 13 after the first flash to about 5 when averaged over three flashes. In the absence of ADP, protons slipped through F0F1. The proton transfer through F0F1 after the first flash contained two kinetic components, of about 6 ms and 20 ms both under the ATP synthesis conditions and under slip. The slower component of proton transfer was substantially suppressed in the absence of ADP. We attribute our observations to the mechanism of energy storage in the ATP-synthase needed to couple the transfer of four protons with the synthesis of one molecule of ATP. Most probably, the transfer of initial protons of each tetrad creates a strain in the enzyme that slows the translocation of the following protons.

Adenosine Diphosphate↗

PubMed1984

Visual pigments and environmental light.

The visual pigments in the rods do not have a special absorption that gives them maximal sensitivity. The visual pigments of "deep sea" fish are an exception for these do match the environmental light to give maximum sensitivity. At the low light intensities at which the rods operate, it is the number of photons that go to make up each element of the image that limits the ability of the eye to discriminate detail and contrast. Chemically induced isomerisation of the visual pigment molecule may cause spurious visual signals that limit the ability of the eye to detect contrasts in very dim light. In bright light the spurious visual signals become insignificant in number compared to the true photon-induced visual signals. Compared to the rods, cone visual pigments do match the spectral properties of the environment except that there appear to be no visual pigments with an absorption maximum beyond the 625 nm porphyropsins in cones. U.V. absorbing pigments are know in invertebrates, birds and fish that live in very shallow water. Animals have photoreceptors in parts of the body other than the eyes. In vertebrates these sites include the pineal, chromatophores, brain, skin and harderian gland. There is evidence based on immunocytochemistry and action spectra that at least some of the skin and pineal receptors contain visual pigments, but like those of the rods, these do not match the spectral quality of the environmental light.

Animals↗

PubMed2003

Regional variability in mottled subclinical melanoderma in the elderly.

The density in melanin chromatophores becomes heterogeneous in the epidermis during the early events of photoaging. Little is known about the regional variability in the resulting mottled skin appearance on the sun-exposed parts of the body in the elderly. The relationship between these features and the dermal atrophy related to aging is also an area ripe for study. The aim of the present study was to objectively assess and compare such aspects of photoaging in older subjects. A computer-assisted video camera equipped with an internal ultraviolet-emitting unit (Visioscan) was used in combination with image analysis to quantify the infraclinical mottling of the skin on the forehead, neck and dorsal forearm. A Densi Score device served to assess the dermal atrophy by rating the skin folding capacity. A mottled subclinical melanoderma was disclosed on the three evaluation sites in all subjects irrespective of their phototypes. Three main patterns were identified corresponding to regular perifollicular dots, streaky macules along shallow wrinkles and unevenly shaped macules in the interfollicular area. The extent in melanoderma was larger on the forehead than on the neck and forearm. The age after 60 years did not influence the severity of mottled melanoderma, but clearly altered the skin folding capacity. The greatest inter-individual differences in mottled melanoderma were found in phototype II subjects. In conclusion, infraclinical mottled melanoderma is present in all individuals after 60 years of age. This type of epidermal photoaging does not appear to be influenced in older ages. By contrast, skin laxity due to dermal aging continues to progress beyond the age of 60 years.

Aged↗

PubMed2003

The structure and evolution of the melanocortin and MCH receptors in fish and mammals.

Zebrafish are an excellent genetic model system for studying developmental and physiological processes. Pigment patterns in zebrafish are affected by mutations in three types of chromatophores. The behavior of these cells is influenced by alpha-melanocyte-stimulating hormone (alphaMSH) and melanin-concentrating hormone (MCH). Mammals have five alphaMSH receptors (melanocortin receptors) and one or two MCH receptors. We have identified the full complement of melanocortin and MCH receptors in both zebrafish and the pufferfish, Fugu. Zebrafish have six melanocortin receptors, including two MC5R orthologues, while Fugu, lacking MC3R, has only four. We also demonstrate that Fugu and zebrafish have two and three MCHR genes, respectively. MC2R and MC5R are physically linked in all species examined. Unlike other species, we find the Fugu genes contain introns, one of which is in a conserved location and is probably ancestral. We also detail the differential expression of the zebrafish genes throughout development.

Animals↗

PubMed2002

Biosensing of opioids using frog melanophores.

Spectacular color changes of fishes, frogs and other lower vertebrates are due to the motile activities of specialized pigment containing cells. Pigment cells are interesting for biosensing purposes since they provide an easily monitored physiological phenomenon. Melanophores, containing dark brown melanin pigment granules, constitute an important class of chromatophores. Their melanin-filled pigment granules may be stimulated to undergo rapid dispersion throughout the melanophores (cells appear dark), or aggregation to the center of the melanophores (cells appear light). This simple physiological response can easily be measured in a photometer. Selected G protein coupled receptors can be functionally expressed in cultured frog melanophores. Here, we demonstrate the use of recombinant frog melanophores as a biosensor for the detection of opioids. Melanophores were transfected with the human opioid receptor 3 and used for opiate detection. The response to the opioid receptor agonist morphine and a synthetic opioid peptide was analyzed by absorbance readings in an aggregation assay. It was shown that both agonists caused aggregation of pigment granules in the melanophores, and the cells appeared lighter. The pharmacology of the expressed receptors was very similar to its mammalian counterpart, as evidenced by competitive inhibition by increasing concentrations of the opioid receptor inhibitor naloxone. Transfection of melanophores with selected receptors enables the creation of numerous melanophore biosensors, which respond selectively to certain substances. The melanophore biosensor has potential use for measurement of substances in body fluids such as saliva, blood plasma and urine.

Analgesics, Opioid↗

PubMed2000

Alpha(1)- and alpha(2)-adrenoceptor mediation in melanosome aggregation in cryptic patterning of Pleuronectes americanus.

The dark upper (ocular) surface of winter flounder (Pleuronectes americanus) possesses a chromatophore pattern, with cryptic dark bands and white spots, and a general background component, with melanophores under adrenergic neural control. In vitro responsiveness to the alpha-adrenoceptor agonists phenylephrine and clonidine and to the antagonists yohimbine and prazosin demonstrates that melanosome aggregation in this species is mediated through both alpha(1)- and alpha(2)-adrenoceptors, the alpha(2)-subtype being predominant in each pattern component. However, differences in the potency of agonists and antagonists indicate variability in the balance between the two receptors between pattern components. This paper demonstrates that it cannot be generalized that teleost melanophore alpha-adrenoceptors are universally of one subtype, and also that there is intraspecific variation in subtypes of these receptors associated with the flatfish cryptic patterning mechanism.

Adrenergic alpha-Agonists↗

PubMed1995

Electron transfer from the tetraheme cytochrome to the special pair in the Rhodopseudomonas viridis reaction center: effect of mutations of tyrosine L162.

The structure of the photosynthetic reaction center (RC) from Rhodopseudomonas viridis is known to high resolution. It contains a firmly bound tetraheme cytochrome from which electrons are donated to a special pair (P) of bacteriochlorophylls, which is photooxidized upon absorption of light. Tyrosine at position 162 of the L-subunit of the reaction center (L 162 Y) is a highly conserved residue positioned halfway between P and the proximal heme group (c-559) of the cytochrome. By specific mutagenesis this residue was exchanged against the amino acids phenylalanine (F), glycine (G), methionine (M), leucine (L), tryptophan (W), threonine (T), and histidine (H). All mutants were expressed in Rps. viridis using a recently established transformation system [Laussermair & Oesterhelt (1992) EMBO J. 11, 777-783]. They were shown biochemically to synthesize all four subunits of the RC (cytochrome, subunits L, M, and H) and to assemble them correctly into the membrane. The structures of two mutants (L 162 F and L 162 T) were determined and found not to differ significantly from the wild-type structure. All mutants grew photosynthetically. The absorption spectrum of all the mutants is the same as in WT, but the redox potential of P and of c-559 was changed by the mutations. The kinetics of electron transfer from the heme group to the special pair were measured in chromatophores by flash absorption. As found earlier in the wild type (Y) several exponential components were needed to fit the data.(ABSTRACT TRUNCATED AT 250 WORDS)

Circular Dichroism↗

PubMed1995

Role of PufX protein in photosynthetic growth of Rhodobacter sphaeroides. 1. PufX is required for efficient light-driven electron transfer and photophosphorylation under anaerobic conditions.

The pufX gene is essential for photoheterotrophic growth of the purple bacterium Rhodobacter sphaeroides. In order to analyze the molecular function of the PufX membrane protein, we constructed a chromosomal pufX deletion mutant and phenotypically compared it to a pufX+ control strain and to two suppressor mutants which are able to grow photosynthetically in the absence of pufX. Using this genetic background, we confirmed that PufX is required for photoheterotrophic growth under anaerobic conditions, although all components of the photosynthetic apparatus were present in similar amounts in all strains investigated. We show that the deletion of PufX is not lethal for illuminated pufX- cells, suggesting that PufX is required for photosynthetic cell division. Since chromatophores isolated from the pufX- mutant were found to be unsealed vesicles, the role of PufX in photosynthetic energy transduction was studied in vivo. We show that PufX is essential for light-induced ATP synthesis (photophosphorylation) in anaerobically incubated cells. Measurements of absorption changes induced by a single turnover flash demonstrated that PufX is not required for electron flow through the reaction center and the cytochrome bc1 complex under anaerobic conditions. During prolonged illumination, however, PufX is essential for the generation of a sufficiently large membrane potential to allow photosynthetic growth. These in vivo results demonstrate that under anaerobic conditions PufX plays an essential role in facilitating effective interaction of the components of the photosynthetic apparatus.

Adenosine Triphosphate↗

PubMed1995

Role of the PufX protein in photosynthetic growth of Rhodobacter sphaeroides. 2. PufX is required for efficient ubiquinone/ubiquinol exchange between the reaction center QB site and the cytochrome bc1 complex.

The PufX membrane protein is essential for photosynthetic growth of Rhodobacter sphaeroides because it is required for multiple-turnover electron transfer under anaerobic conditions [see accompanying article; Barz, W. P., Francia, F., Venturoli, G., Melandri, B. A., Verméglio, A., & Oesterhelt, D. (1995) Biochemistry 34, 15235-15247]. In order to understand the molecular role of PufX, light-induced absorption spectroscopy was performed using a pufX- mutant, a pufX+ strain, and two suppressor mutants. We show that the reaction center (RC) requires PufX for its functionality under different redox conditions than the cytochrome bc1 complex: When the kinetics of flash-induced reduction of cytochrome b561 were monitored in chromatophores, we observed a requirement of PufX for turnover of the cytochrome bc1 complex only at high redox potential (Eh > 140 mV), suggesting a function of PufX in lateral ubiquinol transfer from the RC. In contrast, PufX is required for multiple turnover of the RC only under reducing conditions: When the Q pool was partially oxidized in vivo using oxygen or electron acceptors like dimethyl sulfoxide or trimethylamine N-oxide, the deletion of PufX had no effect on light-driven electron flow through the RC. Flash train experiments under anaerobic in vivo conditions revealed that RC photochemistry does not depend on PufX for the first two flash excitations. Following the third and subsequent flashes, however, efficient charge separation requires PufX, indicating an important role of PufX for fast Q/QH2 exchange at the QB site of the RC. We show that the Q/QH2 exchange rate is reduced approximately 500-fold by the deletion of PufX when the Q pool is nearly completely reduced, demonstrating an essential role of PufX for the access of ubiquinone to the QB site. The fast ubiquinone/ubiquinol exchange is partially restored by suppressor mutations altering the macromolecular antenna structure. These results suggest an indirect role of PufX in structurally organizing a functional photosynthetic apparatus.

Bacterial Proteins↗

PubMed2000

Proteolytic cleavage of the Fe-S subunit hinge region of Rhodobacter capsulatus bc(1) complex: effects of inhibitors and mutations.

The three-dimensional structure of the mitochondrial bc(1) complex reveals that the extrinsic domain of the Fe-S subunit, which carries the redox-active [2Fe2S] cluster, is attached to its transmembrane anchor domain by a short flexible hinge sequence (amino acids D43 to S49 in Rhodobacter capsulatus). In various structures, this extrinsic domain is located in different positions, and the conformation of the hinge region is different. In addition, proteolysis of this region has been observed previously in a bc(1) complex mutant of R. capsulatus [Saribas, A. S., Valkova-Valchanova, M. B., Tokito, M., Zhang, Z., Berry E. A., and Daldal, F. (1998) Biochemistry 37, 8105-8114]. Thus, possible correlations between proteolysis, conformation of the hinge region, and position of the extrinsic domain of the Fe-S subunit within the bc(1) complex were sought. In this work, we show that thermolysin, or an endogenous activity present in R. capsulatus, cleaves the hinge region of the Fe-S subunit between its amino acid residues A46-M47 or D43-V44, respectively, to yield a protease resistant fragment with a M(r) of approximately 18 kDa. The cleavage was affected significantly by ubihydroquinone oxidation (Q(o)) and ubiquinone reduction (Q(i)) site inhibitors and by specific mutations located in the bc(1) complex. In particular, using either purified or detergent dispersed chromatophore-embedded R. capsulatus bc(1) complex, we demonstrated that while stigmatellin blocked the cleavage, myxothiazol hardly affected it, and antimycin A greatly enhanced it. Moreover, mutations in various regions of the Fe-S subunit and cyt b subunit changed drastically proteolysis patterns, indicating that the structure of the hinge region of the Fe-S subunit was modified in these mutants. The overall findings establish that protease accessibility of the Fe-S subunit of the bc(1) complex is a useful biochemical assay for probing the conformation of its hinge region and for monitoring indirectly the position of its extrinsic [2Fe2S] cluster domain within the Q(o) pocket.

Animals↗

PubMed1993

The reaction center associated tetraheme cytochrome subunit from Chromatium vinosum revisited: a reexamination of its EPR properties.

The heme components of chromatophore membranes from the purple bacterium Chromatium vinosum have been studied by EPR. Five different heme species could be distinguished on the basis of their g values, redox midpoint potentials, and orientations of heme planes with respect to the membrane plane: gz = 2.94, Em = +10 mV, 40 degrees-50 degrees; gz = 2.94, Em = +10 mV, 0 degree; gz = 3.1, Em = +330 mV, 90 degrees; gz = 3.3, Em = 360 mV, 30 degrees; gz = 3.4, Em = 0 mV, no detectable orientation. Four of these five hemes (gz = 3.3, gz = 3.1, and 2x gz = 2.94) were ascribed to the tetraheme cytochrome subunit associated with the photosynthetic reaction center of this bacterium. Some of the results obtained have already been reported previously [Tiede, D.M., Leigh, J.S., & Dutton, P.L. (1978) Biochim. Biophys. Acta 503, 524-544] and have led to a model for the tetraheme cytochrome subunit in Chromatium which is significantly different from the three-dimensional structure of the reaction center associated subunit in the purple bacterium Rhodopseudomonas viridis. The additional data obtained in our work, however, require a reinterpretation of the previously published results. The model arrived at is in general agreement with the X-ray structure from Rhodopseudomonas viridis. A model rationalizing the detailed differences between the structure of the Rhodopseudomonas viridis cytochrome subunit and the data obtained on tetraheme subunits from other photosynthetic bacteria is presented.

Chromatium↗

PubMed1991

Study of QB- stabilization in herbicide-resistant mutants from the purple bacterium Rhodopseudomonas viridis.

The pH dependences of the rate constants of P+QB- (kBP) and P+QA- (kAP) charge recombination decays have been studied by flash-induced absorbance change technique, in chromatophores of three herbicide-resistant mutants from Rhodopseudomonas (Rps.) viridis, and compared to the wild type. P, QA, and QB are the primary electron donor and the primary and the secondary quinone acceptors, respectively. The triazine resistant mutants T1 (Arg L217----His and Ser L223----Ala), T3 (Phe L216----Ser and Val M263----Phe), and T4 (Tyr L222----Phe), all mutated in the QB binding pocket of the reaction center, have previously been characterized (Sinning, I., Michel, H., Mathis, P., & Rutherford, A. W. (1989) Biochemistry 28, 5544-5553). The pH dependence curves of kBP in T4 and the wild type are very close. This confirms that the sensitivity toward DCMU of T4 is mainly due to a structural rearrangement in the QB pocket rather than to a change in the charge distribution in this part of the protein. In T3, a 6-fold increase of kAP is observed (kAP = 4200 +/- 300 s-1 at pH 8) compared to that of the wild type (kAP = 720 +/- 50 s-1 at pH 8). We propose that the Val M263----Phe mutation induces a free energy decrease between P+QA- and P+I- (delta G zero IA) (I is the primary electron acceptor) of about 49 meV. The very different pH dependence of kAP in T3 suggests a substantial change in the QA pocket. The 2.5 times increase of kAP above pH 9.5 in the wild type is no longer detected in T3.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug Resistance, Microbial↗

PubMed1991

Kinetic properties of the acceptor quinone complex in Rhodopseudomonas viridis.

The kinetics of electron transfer from the primary (QA) to the secondary (QB) quinone acceptor in whole cells and chromatophores of Rhodopseudomonas viridis was studied as a function of the redox state of QB and of pH by using a photovoltage technique. Under conditions where QB was oxidized, the reoxidation of QA- was found to be essentially monophasic and independent of pH with a half-time of about 20 microseconds. When QB was reduced to the semiquinone form by a preflash, the reoxidation of QA- was slowed down showing a half-time between 40 and 80 microseconds at pH less than or equal to 9. Above pH 9, the rate of the second electron transfer decreased nearly one order of magnitude per pH unit. After a further preflash, the fast and pH-independent kinetics of QA- reoxidation was essentially restored. The concentration of QA still reduced 100 microseconds after its complete reduction by a flash showed distinct binary oscillations as a function of the number of preflashes, confirming the interpretation that the electron-transfer rate depends on the redox state of QB. After addition of o-phenanthroline, the reoxidation of QA- is slowed down to the time range of seconds as expected for a back-reaction with oxidized cytochrome. Under conditions where inhibitors of the electron transfer between the quinones fail to block this reaction in a fraction of the reaction centers due to the presence of the extremely stable and strongly bound semiquinone, QB-, these reaction centers show a slow electron transfer on the first flash and a fast one on the second, i.e., an out-of-phase oscillation.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzoquinones↗