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Biomedical subjects

A Trebst

Publications and source records attributed to A Trebst.

At least 19 recordsLinked to original sources
PubMed2001

Mode of action of new diethylamines in lycopene cyclase inhibition and in photosystem II turnover.

The new bleaching herbicidal compound N,N-diethyl-N-(2-undecynyl)amine (NDUA) is identified here as an inhibitor of lycopene cyclase and is compared with the known cyclase inhibitors N,N-diethyl-N-[2-(4-chlorophenylthio)ethyl]amine (CPTA) and N,N-diethyl-N-[2-(4-methylphenoxy)ethyl]amine (MTPA). HPLC separation of chloroplast pigments shows lycopene accumulation in NDUA treated tissue. Variation in chain length of the undecynylamine moeity of NDUA from 7 to 21 C atoms reveals an optimum of 11 to 14 C atoms for herbicidal activity. A series of seven further analogues of NDUA and CPTA reveals the structural elements necessary for inhibition of lycopene cyclase. The effect of NDUA derivatives on photosynthesis has been studied in Chlamydomonas reinhardtii. Photosynthesis is highly sensitive, particularly towards the C14 and longer chain length analogues at nanomolar concentrations. It is shown that the breakdown of photosynthesis by NDUA is due to interference with the turnover of the D1 protein of the photosystem II reaction centre that requires the continuous biosynthesis of the two reaction-centre beta-carotene moieties in the reassembly phase. The D1 protein disappearance is most marked under strong light conditions. The depletion of photosystem II occurs before total pigment bleaching. This newly recognized mechanism in herbicidal activity is also the basis for the mode of action of other lycopene cyclase inhibitors as well as phytoene desaturase inhibitors.

Animals↗
PubMed1998

Beta-carotene to zeaxanthin conversion in the rapid turnover of the D1 protein of photosystem II.

The carotenoid composition was investigated during enhanced D1 protein turnover in Chlamydomonas reinhardtii exposed to high light. After 2 h of high light there was no loss of the D1 protein yet. However, the beta-carotene content was significantly reduced. In parallel, an increase of the zeaxanthin content was found, which was higher than can be accounted for by the light-induced de-epoxidation of violaxanthin in the xanthophyll cycle reactions. We therefore assume that beta-carotene of photosystem II (PS II) is hydroxylated to zeaxanthin under high light stress. Inhibitors of carotene biosynthesis led to the loss of both PS II activity and D1 protein, indicating the requirement of beta-carotene synthesis for the reassembly of PS II in high light. Diuron blocked D1 protein as well as beta-carotene turnover. In the presence of chloramphenicol -- which allows just one turnover of the D1 protein -- 15% of the total beta-carotene was lost, calculated to be two beta-carotene.

Animals↗
PubMed1997

Role of carotene in the rapid turnover and assembly of photosystem II in Chlamydomonas reinhardtii.

Inhibitors of the phytoene desaturase in carotene biosynthesis were tested in the enhanced rapid turnover of the D1 protein of photosystem II in high light exposure of Chlamydomonas reinhardtii cells. After 1 h high light on heterotrophically grown cells in the presence of norflurazon or fluridone, photosynthesis activity in vivo and PS II activity in vitro is lost. The D1 protein has disappeared. PS I activity is not affected, nor is the D2 protein. It is concluded that beta-carotene is essential for the assembly of the D1 protein into functional photosystem II. It is suspected that bleaching of beta-carotene in the reaction center of PS II by high light destabilizes the structure and triggers the degradation of the D1 protein.

Animals↗
PubMed1994

Molecular characterization of two spontaneous antimycin A resistant mutants of Rhodospirillum rubrum.

Antimycin A is an inhibitor of cytochrome bc1 complexes acting at the quinone reducing site (Qi) of the cytochrome b subunit. We report here the isolation and molecular characterization of two spontaneous mutants of the purple non-sulfur bacterium Rhodospirillum rubrum resistant to this inhibitor. In the two mutants antimycin A resistance was found to be conferred by replacement of an aspartate residue at position 243 of the cytochrome b polypeptide chain, in one case by histidine and in the other by glutamate. The mutants exhibit cross-resistance to aurachin C but not to aurachin D. The exchange of Asp-243 does not only diminish the antimycin sensitivity of the isolated cytochrome bc1 complexes but also has effects on the function of the quinone reducing site (Qi). Oxidant-induced reduction of cytochrome b, requiring addition of antimycin A in the wild type, is already at a maximum in the absence of antimycin A. This indicates a diminished electron flow between heme b-566 and ubiquinone at the quinone reducing site (Qi) of cytochrome b.

Amino Acid Sequence↗
PubMed1993

Engagement of specific sites in the plastoquinone niche regulates degradation of the D1 protein in photosystem II.

Rapid degradation of the photosystem-II reaction center protein, D1, is driven by visible and ultraviolet irradiance at low photon-flux in the intact plant. We find that all inhibitors of electron flow that replace bound plastoquinone (QB) from its niche on D1 inhibit UV-B-driven D1 degradation, but only some inhibit visible light-driven degradation. Stabilization of the D1 protein by nitrophenol type inhibitors in visible light depends on the dimensions of the side chain at position 6 of the phenyl ring. Likewise, resistance to trypsinization of D1 at Arg-238 and to electron flow inhibitors in D1 mutant V219I (but not A251V, F255Y, S264A, or L275F) are functions of position 6 side chain dimensions in isolated thylakoids. We conclude that the configuration of the QB niche controls D1 protein degradation in intact plants under physiological photon flux. This is true irrespective of the spectral quality of radiation driving degradation. We show that UV-B-driven D1 protein degradation, but not that driven by visible light, requires plastoquinone in the QB niche to proceed. D1 degradation in visible light occurs as long as specific regions at the end of helix D and in the D-de loop of the protein are not engaged. These regions, through substrate (i.e. QB)-mediated stabilization, are proposed to regulate rapid degradation of the D1 protein.

Animals↗
PubMed1992

A new way to monitor by-pass restorations of electron transport in inhibited chloroplasts by cyclic electron flow cofactors--a study by modulated fluorimetry.

Inhibition of electron transport in broken chloroplasts by DBMIB, under light-limiting conditions, is shown to be bypassed by PMS in a manner similar to the known effects of the phenylenediamine derivatives TMPD and DAD. These bypasses were demonstrated and further studied by modulated fluorimetry, monitoring DBMIB inhibition by the shift of the steady-state fluorescence towards the Fm level and the release of inhibition by a reverse shift together with establishment of a quenching effect by background far-red light. Comparative studies were also made with electron transport blocked by DCMU or BNT. A weak bypass by TMPD and a weaker one by PMS of the block created by DCMU was observed by modulated fluorimetry. The block created by BNT is similarly shown to be bypassed by TMPD but hardly or not at all by PMS. Bypass effects persisted even in the presence of ascorbate. It appears that, following reduction of the different cofactors by ascorbate in the stroma side, illumination caused the accumulation of a pool of oxidized cofactor molecules in the lumen, which is able to mediate electron transport between reduced plastoquinone and plastocyanin or P-700. The existence and the size of this pool were found to depend largely on the internal pH at the lumen, presenting an artificial system in which electron flow is controlled by the lumenal pH. The bypassing electron transport in the presence of DBMIB presumably avoids the participation of the cytochrome b6f complex. During its occurrence, there is also a strong imbalance in the activities of the two photosystems for linear electron flow, in favor of PS II. These experiments may thus serve to establish an in vitro model system for a future investigation of effects related to changes in the imbalance between the two photosystems and its regulation. Furthermore, this experimental system may also be utilized to study the role of the internal lumenal pH in control of photosynthesis.

Ascorbic Acid↗
PubMed1991

Site-specific mutagenesis of the D1 subunit of photosystem II in wild-type Chlamydomonas.

The structure and functional mode of photosystem II reaction center protein D1 can be studied by analyzing the effects of amino acid substitutions within the binding niche for QB, the second stable electron acceptor of photosystem II, on herbicide binding. Here we report on site-directed mutagenesis of the psbA gene coding for the D1 protein in the unicellular alga Chlamydomonas reinhardtii. The chloroplasts of wild-type cells were transformed using the particle gun. The plasmids introduced carried an in vitro mutated fragment of the psbA gene. We obtained a double mutant with replacements of amino acids 264 and 266 and a triple mutant having an additional substitution in position 259. The sensitivities of both mutants toward several types of herbicides are given and compared with those of a mutant having only a substitution at position 264.

Amino Acid Sequence↗
PubMed1990

The pet genes of Rhodospirillum rubrum: cloning and sequencing of the genes for the cytochrome bc1-complex.

A cytochrome bc1-complex of Rs. rubrum was isolated and the three subunits were purified to homogeneity. The N-terminal amino acid sequence of the purified subunits was determined by automatic Edman degradation. The pet genes of Rhodospirillum rubrum coding for the three subunits of the cytochrome bc1-complex were isolated from a genomic library of Rs. rubrum using oligonucleotides specific for conserved regions of the subunits from other organisms and a heterologous probe derived from the genes for the complex of Rb. capsulatus. The complete nucleotide sequence of a 5500 bp SalI/SphI fragment is described which includes the pet genes and three additional unidentified open reading frames. The N-terminal amino acid sequence of the isolated subunits was used for the identification of the three genes. The genes encoding the subunits are organized as follows: Rieske protein, cytochrome b, cytochrome c1. Comparison of the N-terminal protein sequences with the protein sequences deduced from the nucleotide sequence showed that only cytochrome c1 is processed during transport and assembly of the three subunits of the complex. Only the N-terminal methionine of the Rieske protein is cleaved off. The similarity of the deduced amino acid sequence of the three subunits to the corresponding subunits of other organisms is described and implications for structural features of the subunits are discussed.

Amino Acid Sequence↗
PubMed1984

Sequence homology and structural similarity between cytochrome b of mitochondrial complex III and the chloroplast b6-f complex: position of the cytochrome b hemes in the membrane.

The amino acid sequences of cytochrome b of complex III from five different mitochondrial sources (human, bovine, mouse, yeast, and Aspergillus nidulans) and the chloroplast cytochrome b6 from spinach show a high degree of homology. Calculation of the distribution of hydrophobic residues with a "hydropathy" function that is conserved in this family of proteins implies that the membrane-folding pattern of the 42-kilodalton (kDa) mitochondrial cytochromes involves 8-9 membrane-spanning domains. The smaller 23-kDa chloroplast cytochrome appears to fold in five spanning domains that are similar to the first five of the mitochondria. Four highly conserved histidines are considered to be the likely ligands for the two hemes. The positions of the histidines along the spanning segments and in a cross section of the membrane-spanning alpha helices implies that two ligand pairs, His-82-His-197/198 and His-96-His-183, bridge the spanning peptides II and V, and the two hemes reside on opposite sides of the hydrophobic membrane core. In addition, the 17-kDa protein of the chloroplast b6-f complex appears to contain one or more of the functions of the COOH-terminal end of the mitochondrial cytochrome b polypeptide.

Amino Acid Sequence↗
PubMed1982

Localization of electron transport inhibition in plastoquinone reactions.

Reduction kinetics of P700 following a short flash are measured in spinach chloroplasts after oxidation of the electron carriers between the two photoreactions by far-red light. Three features of the kinetics allow us to localize simultaneously inhibition at different sites between photoreaction II and the reducing site of plastoquinol. These are the initial lag, the halftime and the area under the transient of the P700 absorbance change, which indicate the electron transfer time from photoreaction II to the reducing site of plastoquinol, the rate of plastoquinol oxidation, and the number of electrons transferred to the special plastoquinone B functioning as secondary electron acceptor of photosystem II, respectively. As an additional diagnostic parameter for inhibition before and after the plastoquinone pool, the area under the transient of the P700 absorbance change is used after long flashes. This area is proportional to the amount of reduced plastoquinone as shown by the absorbance change at 265 nm. The effects of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) are compared with those of 2-bromo-4-nitrothymol, 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol, and Illoxan as representatives for new classes of inhibitors. While 2-halogeno-4-nitrothymols inhibit the reduction of plastoquinone similarly to DCMU, their diphenyl ether derivatives inhibit selectively the oxidation of plastoquinol.

Chlorophyll↗