Photosynthesis

About: Photosynthesis is a research topic. Over the lifetime, 19789 publications have been published within this topic receiving 895197 citations.

A metalloradical mechanism for the generation of oxygen from water in photosynthesis

Abstract: In plants and algae, photosystem II uses light energy to oxidize water to oxygen at a metalloradical site that comprises a tetranuclear manganese cluster and a tyrosyl radical. A model is proposed whereby the tyrosyl radical functions by abstracting hydrogen atoms from substrate water bound as terminal ligands to two of the four manganese ions. Molecular oxygen is produced in the final step in which hydrogen atom transfer and oxygen-oxygen bond formation occur together in a concerted reaction. This mechanism establishes clear analogies between photosynthetic water oxidation and amino acid radical function in other enzymatic reactions.

Regulation of protein metabolism: Coupling of photosynthetic electron transport to in vivo degradation of the rapidly metabolized 32-kilodalton protein of the chloroplast membranes

Abstract: In Spirodela oligorrhiza, mature chloroplasts copiously synthesize and degrade a 32-kilodalton membrane protein. The rates of synthesis and degradation are controlled by light intensity, the protein being unstable in the light and stable in the dark. Light-driven synthesis, but not degradation, is dependent on ATP. Degradation is blocked by herbicides inhibiting photosystem II electron transport, such as diuron and atrazine. Thus, both anabolism and catabolism of the 32-kilodalton protein are photoregulated, with degradation coupled to electron transport rather than phosphorylation.

Ethanol Synthesis by Genetic Engineering in Cyanobacteria

Abstract: Cyanobacteria are autotrophic prokaryotes which carry out oxygenic photosynthesis and accumulate glycogen as the major form of stored carbon. In this research, we introduced new genes into a cyanobacterium in order to create a novel pathway for fixed carbon utilization which results in the synthesis of ethanol. The coding sequences of pyruvate decarboxylase (pdc) and alcohol dehydrogenase II (adh) from the bacterium Zymomonas mobilis were cloned into the shuttle vector pCB4 and then used to transform the cyanobacterium Synechococcus sp. strain PCC 7942. Under control of the promoter from the rbcLS operon encoding the cyanobacterial ribulose-1,5-bisphosphate carboxylase/oxygenase, the pdc and adh genes were expressed at high levels, as demonstrated by Western blotting and enzyme activity analyses. The transformed cyanobacterium synthesized ethanol, which diffused from the cells into the culture medium. As cyanobacteria have simple growth requirements and use light, CO2, and inorganic elements efficiently, production of ethanol by cyanobacteria is a potential system for bioconversion of solar energy and CO2 into a valuable resource. Cyanobacteria, also known as blue-green algae, are autotrophic prokaryotes which exhibit diversity in metabolism, structure, morphology, and habitat. However, all of these organisms perform oxygenic photosynthesis, and this photosynthesis is similar to that performed by higher plants (29, 32). As the cyanobacteria have simple growth requirements, grow to high densities, and use light, carbon dioxide, and other inorganic nutrients efficiently, they could be attractive hosts for production of valuable organic products. In fact, many cyanobacteria can be used directly as food and fodder since they are nonpathogenic and have high nutrient value (27). Some cyanobacteria also synthesize secondary metabolites which have been reported to have significant therapeutic effects (4). In addition, mass cultivation for commercial production of some cyanobacteria can be performed efficiently. Synechococcus sp. strain PCC 7942 (previously referred to as Anacystis nidulans R2), a unicellular cyanobacterium that lives in freshwater, is one of the few cyanobacterial strains which have been relatively well-characterized in terms of physiology, biochemistry, and genetics. This organism is able to take up foreign DNA and can be transformed either by using shuttle vectors capable of replicating in both Escherichia coli and the cyanobacterium or by integrating foreign DNA into the chromosome through homologous recombination at targeted sites (14, 36). In recent years, workers have achieved limited success in expressing foreign genes in this cyanobacterium, as well as other transformable strains. For example, the human carbonic anhydrase gene caII used to investigate CO2-concentrating mechanisms (26), E. coli and human superoxide dismutase genes used to investigate oxidative stress (15, 34), E. coli pet genes used to increase salt stress resistance (25), and Bacillus thuringiensis larvicidal genes used to develop bioinsecticidal hosts (33, 35) have all been expressed in Synechococcus sp. at sufficiently high levels to generate discernible phenotypes. In this paper, we describe our attempts to transform Synechococcus sp. strain PCC 7942 with bacterial genes in order to create a novel pathway for ethanol production in cyanobacteria.

Inhibition of photosynthesis by heavy metals.

Abstract: Inhibition of photosynthesis by heavy metals is well documented. In this review the results are compared between in vitro experiments on isolated systems (chloroplasts, enzymes -.), experiments on excised leaves and intact plants and algae in vivo. In vitro experiments suggest potential sites of heavy metal interaction with photosynthesis at several levels of organisation, which are not necessarily confirmed in vivo. Analytical data on subcellular heavy metal level are generally missing to discuss their mechanism of action in the intact organism. In the field factors such as soil characteristics and air pollution have to be taken into account for assessing the mechanism of action of heavy metals on photosynthesis in plants, growing in a polluted erea.