Biomedical subjects

P Horton

Publications and source records attributed to P Horton.

At least 19 recordsLinked to original sources

PubMed2006

The effect of fertility control on the transmission of bovine tuberculosis in wild brushtail possums.

AIM: To determine the effect of fertility control on the rate of transmission of bovine tuberculosis (Tb), caused by Mycobacterium bovis, in brushtail possums (Trichosurus vulpecula). METHODS: At two study sites with a history of Tb infection in the resident possum population, a sample of adult male and female possums (n=50), determined by palpation to be Tb-free, was surgically sterilised by gonadectomy. A sample of untreated Tb-free male and female possums (n=54) served as controls. Each study site was trapped every 2 months over a 3-year period, and the Tb status of the individuals in the trial assessed. At the conclusion of the trial, all remaining experimental individuals were killed, necropsied and examined for characteristic lesions typical of Tb. The rate of transmission of Tb infection was estimated using the incidence of tuberculous cases in the experimental animals and comparing it between sites, sex and sterilisation treatment. RESULTS: Sterilisation by gonadectomy resulted in a reduction in the rate of transmission of Tb in male possums by 53%, but a corresponding increase in sterilised females for reasons that are still unclear. However, this interaction was statistically weak (p=0.10). When the sexes were combined, there was no overall effect of sterilisation on the rate of transmission of Tb (p=0.43). Sterility treatment notwithstanding, there was a highly significant difference in the rate of transmission between the sexes (p=0.01), being almost one order of magnitude higher in untreated males compared with untreated females. CONCLUSIONS: Although lacking strong statistical support, these results suggest that fertility control that targets endocrine control of fertility may provide some additional benefits for disease control over that achieved by reductions in population density, by reducing the rate of transmission of M. bovis in male possums. However, additional studies are needed to confirm the validity of these results. The large difference in the rate of transmission of M. bovis in untreated males compared with untreated females suggests that transmission of Tb among males may be an important driver of the dynamics of Tb infection in possums.

Animals↗

PubMed2005

Acclimation of photosynthesis to high irradiance in rice: gene expression and interactions with leaf development.

Rice (Oryza sativa L.) has been used to study the long-term responses of photosynthesis to high irradiance focusing on the composition of the photosynthetic apparatus and leaf morphology. Typical sun/shade differences in chloroplast composition are seen in the fifth leaf following growth in high irradiance compared with low irradiance (1000 and 200 micromol m(-2) s(-1), respectively): higher light-saturated rates of photosynthesis (P(max)), higher amounts of Rubisco protein, and a lower chlorophyll a:b ratio. In addition, leaves were thicker under high light compared with low light. However, responses appear more complex when leaf developmental stage is considered. Using a system of transferring plants from low to high light in the laboratory responses that occur before and after full leaf extension have been studied. Acclimation of photosynthesis is limited by leaf age: the transfer to high light, post-leaf extension, is characterized by alterations in chlorophyll a:b but not in Rubisco protein, which may be limited by leaf morphology. Microarray analysis of gene expression was carried out on plants that were transferred to high light post-leaf extension. A down-regulation of light-harvesting genes was seen. No change in the expression level of Rubisco genes was observed. Up-regulation of genes involved in photoprotection was observed. It was also shown that high-light leaf morphology is established prior to formation of the zone of cellular elongation and division. The endogenous and environmental factors which establish the characteristics of high light acclimation may be important for attaining high rates of assimilation in leaves and crop canopies, and the fifth leaf in rice provides a convenient model system for the determination of the mechanisms involved.

Acclimatization↗

PubMed2003

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.

Arabidopsis↗

PubMed2003

Physiological interpretation of electrical impedance epigastrography measurements.

Measurement of the electrical impedance of the gastric region is carried out with the epigastrograph. This generates and applies alternating current around the abdominal area and measures the potential difference in order to determine the impedance externally, via electrodes. The change of epigastric impedance for a subject, given a meal after fasting, depends on the conductivity of the meal compared to the stomach and surrounding tissues. Typically a conductive meal has conductivity >7 mS cm(-1), non-conductive <2 mS cm(-1) and neutral about 4.5 mS cm(-1). Half-emptying times (T50s) from gastric emptying studies in volunteers using three test meals of 450 ml volume were obtained and found to be shorter than expected from the literature. The meals were a 10% glucose solution and two milk shakes of energy 1,300 kJ and 2,850 kJ, respectively. These electrical impedance epigastrography (EIE) measurements were carried out with scintigraphy. The T50 values of the latter were significantly longer. The direct comparison of the normalized experimental data obtained by both methods led to the concept that EIE measurements are mainly influenced by gastric secretion. Thus the EIE trace of a 'neutral' meal suggests the hypothesis that the volume of the meal is not the significant factor but is influenced by gastric acid secretions. Physiology of the gastric mucosa during the digestion of a meal and intragastric pH values also suggests this. Gastric function studies using EIE measurements may therefore reflect gastric ionic concentration rather than the volume of the contents of the stomach. In turn this could lead to the development of a non-invasive method for the continuous recording of gastric acid secretions.

Beverages↗

PubMed2002

Overexpression of beta-carotene hydroxylase enhances stress tolerance in Arabidopsis.

Plant stress caused by extreme environmental conditions is already a principal reason for yield reduction in crops. The threat of global environment change makes it increasingly important to generate crop plants that will withstand such conditions. Stress, particularly stress caused by increased sunlight, leads to the production of reactive oxygen species that cause photo-oxidative cell damage. Carotenoids, which are present in the membranes of all photosynthetic organisms, help protect against such light-dependent oxidative damage. In plants, the xanthophyll cycle (the reversible interconversion of two carotenoids, violaxanthin and zeaxanthin) has a key photoprotective role and is therefore a promising target for genetic engineering to enhance stress tolerance. Here we show that in Arabidopsis thaliana overexpression of the chyB gene that encodes beta-carotene hydroxylase--an enzyme in the zeaxanthin biosynthetic pathway--causes a specific twofold increase in the size of the xanthophyll cycle pool. The plants are more tolerant to conditions of high light and high temperature, as shown by reduced leaf necrosis, reduced production of the stress indicator anthocyanin and reduced lipid peroxidation. Stress protection is probably due to the function of zeaxanthin in preventing oxidative damage of membranes.

Anthocyanins↗

PubMed2001

Kinetic analysis of nonphotochemical quenching of chlorophyll fluorescence. 1. Isolated chloroplasts.

Nonphotochemical quenching of chlorophyll fluorescence in plants is indicative of a process that dissipates excess excitation energy from the light-harvesting antenna of photosystem II. The major fraction of quenching is obligatorily dependent upon the thylakoid DeltapH and is regulated by the de-epoxidation state of the xanthophyll cycle carotenoids associated with the light-harvesting complexes. Basic principles of enzyme kinetics have been used to investigate this process in isolated chloroplasts. The extent of quenching was titrated against the estimated thylakoid lumen pH, and a sigmoidal relationship was obtained with a Hill coefficient of 4.5 and a pK of 4.7. Upon de-epoxidation, these parameters changed to 1.6 and 5.7, respectively. Antimycin A suppressed quenching, increasing the Hill coefficient and reducing the pK. The rate of induction of quenching fitted second-order kinetics with respect to illumination time, and the rate constant was dependent upon the DeltapH, the de-epoxidation state, the presence of antimycin, and also the presence of dibucaine, a quenching enhancer. All these data are consistent with the notion that quenching is caused by a conformational transition in a chloroplast thylakoid protein; this transition shows cooperativity with respect to proton binding, and is controlled by de-epoxidation state and various exogenous reagents.

Chlorophyll↗

PubMed2001

Kinetic analysis of nonphotochemical quenching of chlorophyll fluorescence. 2. Isolated light-harvesting complexes.

The chlorophyll fluorescence yield of purified photosystem II light-harvesting complexes can be lowered by manipulation of experimental conditions. In several important respects, this quenching resembles the nonphotochemical quenching observed in isolated chloroplasts and leaves, therefore providing a model system for investigating the underlying mechanism. A methodology based on the principles of enzyme kinetic analysis has already been applied to isolated chloroplasts, and this same experimental approach was used here with purified LHCIIb, CP26, and CP29. It was found that the kinetics of the decrease in fluorescence yield robustly fitted a second-order kinetic model with respect to time after induction of quenching. The second-order rate constant was dependent upon the complex that was analyzed, the detergent concentration, the solution pH, and the presence of exogenous xanthophyll cycle carotenoids. In contrast, the formation of an absorbance change at 683 nm that accompanies quenching displayed first-order kinetics. The reversal of quenching also displayed second-order kinetics. These data show that quenching results from a binary reaction, possibly arising between two chlorophyll molecules. On the basis of these data, a model for the regulation of nonphotochemical quenching based upon the allosteric control of the conformation of light-harvesting complexes by protonation and xanthophyll binding is presented.

Chlorophyll↗

PubMed2001

Configuration and dynamics of xanthophylls in light-harvesting antennae of higher plants. Spectroscopic analysis of isolated light-harvesting complex of photosystem II and thylakoid membranes.

Resonance Raman excitation spectroscopy combined with ultra low temperature absorption spectral analysis of the major xanthophylls of higher plants in isolated antenna and intact thylakoid membranes was used to identify carotenoid absorption regions and study their molecular configuration. The major electronic transitions of the light-harvesting complex of photosystem II (LHCIIb) xanthophylls have been identified for both the monomeric and trimeric states of the complex. One long wavelength state of lutein with a 0-0 transition at 510 nm was detected in LHCIIb trimers. The short wavelength 0-0 transitions of lutein and neoxanthin were located at 495 and 486 nm, respectively. In monomeric LHCIIb, both luteins absorb around 495 nm, but slight differences in their protein environments give rise to a broadening of this band. The resonance Raman spectra of violaxanthin and zeaxanthin in intact thylakoid membranes was determined. The broad 0-0 absorption transition for zeaxanthin was found to be located in the 503-511 nm region. Violaxanthin exhibited heterogeneity, having two populations with one absorbing at 497 nm (0-0), 460 nm (0-1), and 429 nm (0-2), and the other major pool absorbing at 488 nm (0-0), 452 nm (0-1), and 423 nm (0-2). The origin of this heterogeneity is discussed. The configuration of zeaxanthin and violaxanthin in thylakoid membranes was different from that of free pigments, and both xanthophylls (notably, zeaxanthin) were found to be well coordinated within the antenna proteins in vivo, arguing against the possibility of their free diffusion in the membrane and supporting our recent biochemical evidence of their association with intact oligomeric light-harvesting complexes (Ruban, A. V., Lee, P. J., Wentworth, M., Young, A. J., and Horton, P. (1999) J. Biol. Chem. 274, 10458-10465).

Carotenoids↗

PubMed2001

Increasing rice photosynthesis by manipulation of the acclimation and adaptation to light.

There are three important considerations in assessing the interaction of crop plants with light: (a) how does the plant respond to the light environment both in the short-term (regulation) and in the long-term (acclimation), (b) under what conditions are these responses inadequate, leading to photoinhibition, and (c) are the responses optimally adapted for maximum agricultural yield? Despite a wealth of knowledge about these processes in model plant species, it is impossible to predict how significant they are in influencing the yield of rice. Therefore, in collaboration with IRRI, we have undertaken a study of photoinhibition and photoacclimation of rice under field conditions. The results of this study are presented, along with an assessment of the implications for improvement of rice yield.

Adaptation, Physiological↗

PubMed2001

Acclimation of Arabidopsis thaliana to the light environment: the existence of separate low light and high light responses.

The capacity for photosynthetic acclimation in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta was assessed during growth over a broad range of irradiance. Discontinuities in the response to growth irradiance were revealed for the light- and CO2-saturated rate of photosynthesis (Pmax) and the ratio of chlorophyll a to chlorophyll b (Chl a/b). Three separate phases in the response of Pmax and Chl a/b to growth light were evident, with increases at low and high irradiance ranges and a plateau at intermediate irradiance. By measuring all chlorophyll-containing components of the thylakoid membrane that contribute to Chl a/b we reveal that distinct strategies for growth at low and high irradiance underlie the discontinuous response. These strategies include, in addition to changes in the major light-harvesting complexes of photosystem II (LHCII), large shifts in the amounts of both reaction centres as well as significant changes in the levels of minor LHCII and LHCI components.

Acclimatization↗

PubMed2001

Crystal structure of a bifunctional transformylase and cyclohydrolase enzyme in purine biosynthesis.

ATIC, the product of the purH gene, is a 64 kDa bifunctional enzyme that possesses the final two activities in de novo purine biosynthesis, AICAR transformylase and IMP cyclohydrolase. The crystal structure of avian ATIC has been determined to 1.75 A resolution by the MAD method using a Se-methionine modified enzyme. ATIC forms an intertwined dimer with an extensive interface of approximately 5,000 A(2) per monomer. Each monomer is composed of two novel, separate functional domains. The N-terminal domain (up to residue 199) is responsible for the IMPCH activity, whereas the AICAR Tfase activity resides in the C-terminal domain (200-593). The active sites of the IMPCH and AICAR Tfase domains are approximately 50 A apart, with no structural evidence of a tunnel connecting the two active sites. The crystal structure of ATIC provides a framework to probe both catalytic mechanisms and to design specific inhibitors for use in cancer chemotherapy and inflammation.

Animals↗

PubMed2001

Tsukuba BB: a branch and bound algorithm for local multiple alignment of DNA and protein sequences.

In this paper we present a branch and bound algorithm for local gapless multiple sequence alignment (motif alignment) and its implementation. The algorithm uses both score-based bounding and a novel bounding technique based on the "consistency" of the alignment. A sequence order independent search tree is used in conjunction with a technique for avoiding redundant calculations inherent in the structure of the tree. This is the first program to exploit the fact that the motif alignment problem is easier for short motifs. Indeed, for a short fixed motif width, the running time of the algorithm is asymptotically linear in the size of the input. We tested the performance of the program on a dataset of 300 E. coli promoter sequences and a dataset of 85 lipocalin protein sequences. For a motif width of 4, the optimal alignment of the entire set of sequences can be found. For the more natural motif width of 6, the program can align 21 sequences of length 100, more than twice the number of sequences which can be aligned by the best previous exact algorithm. The algorithm can relax the constraint of requiring each sequence to be aligned, and align 105 of the 300 promoter sequences with a motif width of 6. For the lipocalin dataset, we introduce a technique for reducing the effective alphabet size with a minimal loss of useful information. With this technique, we show that the program can find meaningful motifs in a reasonable amount of time by optimizing the score over three motif positions.

Algorithms↗

PubMed2001

Antisense inhibition of the photosynthetic antenna proteins CP29 and CP26: implications for the mechanism of protective energy dissipation.

The specific roles of the chlorophyll a/b binding proteins CP29 and CP26 in light harvesting and energy dissipation within the photosynthetic apparatus have been investigated. Arabidopsis was transformed with antisense constructs against the genes encoding the CP29 or CP26 apoprotein, which gave rise to several transgenic lines with remarkably low amounts of the antisense target proteins. The decrease in the level of CP24 protein in the CP29 antisense lines indicates a physical interaction between these complexes. Analysis of chlorophyll fluorescence showed that removal of the proteins affected photosystem II function, probably as a result of changes in the organization of the light-harvesting antenna. However, whole plant measurements showed that overall photosynthetic rates were similar to those in the wild type. Both antisense lines were capable of the qE type of nonphotochemical fluorescence quenching, although there were minor changes in the capacity for quenching and in its induction kinetics. High-light-induced violaxanthin deepoxidation to zeaxanthin was not affected, although the pool size of these pigments was decreased slightly. We conclude that CP29 and CP26 are unlikely to be sites for nonphotochemical quenching.

Amino Acid Sequence↗

PubMed2000

Allosteric regulation of the light-harvesting system of photosystem II.

Non-photochemical quenching of chlorophyll fluorescence (NPQ) is symptomatic of the regulation of energy dissipation by the light-harvesting antenna of photosystem II (PS II). The kinetics of NPQ in both leaves and isolated chloroplasts are determined by the transthylakoid delta pH and the de-epoxidation state of the xanthophyll cycle. In order to understand the mechanism and regulation of NPQ we have adopted the approaches commonly used in the study of enzyme-catalysed reactions. Steady-state measurements suggest allosteric regulation of NPQ, involving control by the xanthophyll cycle carotenoids of a protonation-dependent conformational change that transforms the PS II antenna from an unquenched to a quenched state. The features of this model were confirmed using isolated light-harvesting proteins. Analysis of the rate of induction of quenching both in vitro and in vivo indicated a bimolecular second-order reaction; it is suggested that quenching arises from the reaction between two fluorescent domains, possibly within a single protein subunit. A universal model for this transition is presented based on simple thermodynamic principles governing reaction kinetics.

Allosteric Regulation↗

PubMed2000

Pigment binding site properties of two photosystem II antenna proteins. A resonance raman investigation.

Two light-harvesting proteins associated with photosystem II of higher plants, namely the major antenna complex LHCIIb and the minor Lhcb4 protein (CP29), have been investigated by resonance Raman spectroscopy. One of the two chlorophylls b and up to five of the six chlorophylls a present in Lhcb4 are shown to adopt similar binding conformations to the (presumably) corresponding molecules in LHCIIb, whereas at least two chlorophylls in the former protein assume unique conformations relative to the bulk complex. The overall conformation of bound xanthophyll molecules is identical in the two antenna proteins, although some small differences are apparent. The pigment binding properties of these two LHCs are discussed, with particular reference to possible structural motifs within this extended family of proteins.

Binding Sites↗

PubMed2000

Chlorophyll fluorescence quenching in isolated light harvesting complexes induced by zeaxanthin.

Non-photochemical quenching of chlorophyll fluorescence in plants occurs in the light harvesting antenna of photosystem II and is regulated by the xanthophyll cycle. A new in vitro model for this process has been developed. Purified light harvesting complexes above the detergent critical micelle concentration have a stable high fluorescence yield but a rapidly inducible fluorescence quenching occurs upon addition of zeaxanthin. Violaxanthin was without effect, lutein and antheraxanthin induced a marginal response, whereas the violaxanthin analogue, auroxanthin, induced strong quenching. Quenching was not caused by aggregation of the complexes but was accompanied by a spectral broadening and red shift, indicating a zeaxanthin-dependent alteration in the chlorophyll environment.

Chlorophyll↗

PubMed2000

Electron acceptors in isolated intact spinach chloroplasts act hierarchically to prevent over-reduction and competition for electrons.

Electron fluxes in isolated intact spinach chloroplasts were analyzed under saturating light and under optimal CO(2) and P(i) supply. When CO(2) assimilation was the only ATP- and NADPH-consuming reaction, the DeltapH decreased and the chloroplasts showed clear evidence of over-reduction. This suggested that additional electron flow is required in order to maintain the DeltapH and the stromal NADPH/ATP ratio. The additional electron flow may be cyclic electron transport around Photosystem I and linear electron transport towards either oxaloacetate or O(2). The contributions of, and the interrelationships between, these three electron transfer pathways were analyzed by following the reactions of chloroplasts in their presence or absence, and by monitoring to what extent they were able to compensate for each other. Inhibition of cyclic electron flow by antimycin A caused strong over-reduction and decreased the DeltapH. Only oxaloacetate, but not O(2), was able to restore photosynthesis. In the presence of H(2)O(2), there was a rapid build-up of a high DeltapH, and the reduction of any other electron acceptor was prevented. It is concluded that the different electron acceptors in the stroma are organized in a hierarchical manner; this allows electron flux towards CO(2) and nitrite reduction to proceed without any competition for electrons, and any excess electrons to be taken by these additional non-assimilatory pathways. Hence, the DeltapH is maintained at the required level and over-reduction of the electron transport chain and the stromal redox components is avoided.

Journal Article↗

PubMed2000

Prospects for crop improvement through the genetic manipulation of photosynthesis: morphological and biochemical aspects of light capture.

The prospects for genetic manipulation of photosynthesis are assessed with an emphasis on the biochemical and morphological aspects of light capture. The connection between different parts of the photosynthetic process is considered together with the influence of environmental factors, development and acclimation, and metabolic regulation. The sites of real and potential photosynthetic losses are identified, using tropical rice as a case study. The important interaction between photosynthetic capacity, acclimation to the light environment, nitrogen accumulation and canopy architecture are discussed. The possibility of genetic intervention to increase both biomass accumulation and improve nitrogen economy simultaneously are considered. Finally, the numerous procedures for genetic manipulation of light harvesting are also discussed, with a view of improving radiation-use efficiency in crops.

Biomass↗