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Posted: Jul 10, 2013
Optimizing microbe factories
(Nanowerk News) A joint research project of the Max Planck and Fraunhofer Societies has received substantial financial support enabling scientists to open the door for new discoveries with immediate industrial applications. In the next three years, researchers from the Fraunhofer Institute for Molecular Biology and Applied Ecology, Aachen, and the Max Planck Institute for Chemical Ecology, Jena, will join forces for a common project: the optimization of the MEP pathway. Microbes and plant chloroplasts use this metabolic route to produce a diversity of active compounds including many substances humans have been employing as pharmaceuticals, crop protection compounds and industrial materials for thousands of years. However, the purification or chemical synthesis of these compounds requires extensive efforts. Therefore the goal of the joint project is to utilize bacteria with an optimized MEP pathway to improve the biosynthetic yield of various natural products.
Model plant Arabidopsis thaliana in a tube.
Close relatives: chloroplasts and prokaryotes
The Fraunhofer / Max Planck cooperation is actually based on a natural event that most likely occurred many millions of years ago: In the course of evolution protozoa absorbed other unicellular organisms, such as ancestors of prokaryotic cyanobacteria. According to this theory, the result of this “endosymbiosis” is the development of plants; the cells of all our plant species contain chloroplasts that emerged from ancestral cyanobacteria and still have substantial similarities to free-living prokaryotes.
A few years ago, plant scientists discovered that chloroplasts contain a metabolic pathway which is also found in prokaryotes such as bacteria, that leads to the production of many so-called terpenoid metabolites: the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway, a finding which confirmed the endosymbiont theory. Intermediates of glycolysis are used in the pathway for the multistep synthesis of molecular monomers that consist of five carbon atoms each. These five carbon units can then be combined in many different ways in order to form chlorophyll, carotenoids, cytokinins, sterols and a multitude of other terpenoids. The production of some plant toxins used as defenses against pests is also based on the MEP biosynthetic pathway.
Vitamins and aromas, pharmaceuticals and plant protection compounds: The production of many valuable natural substances is still difficult and expensive.
The discovery of many biologically-active substances in plants and bacteria has proven beneficial to mankind. However, their isolation, purification and processing from natural sources is not only extremely laborious, but also expensive. Chemical synthesis is also difficult or even impossible because these substances usually have complex carbon skeletons that are hard to make in pure forms. For example, the purification of the elementary substance isoprene from mineral oil - a process which is still commonly used today - is neither environmentally friendly, nor economically sustainable. Stefan Jennewein, a scientist at the Fraunhofer Institute for Molecular Biology and Applied Ecology, as well as Louwrance Wright and Jonathan Gershenzon from the Max Planck Institute for Chemical Ecology therefore decided to study the MEP pathway, a metabolic pathway involved in the biosynthesis of the isoprene units, in more detail. They aim to manipulate this pathway in a way that will facilitate a more efficient production of the pharmaceutically relevant substances by using metabolically modified bacteria. The Max Planck scientists will study the regulation of the MEP pathway in chloroplasts of the model plant Arabidopsis thaliana to learn the principles of metabolic control, whereas Stefan Jennewein’s part is to create bacterial strains of the species Escherichia coli and Clostridium ljungdahlii which will be able to produce the required substances in high yield once their MEP pathways have been optimized.
Thanks to a 1.6 million EUR funding for both institutes in the next three years, the regulation of the seven consecutive enzymatic steps, the levels of the metabolic intermediates, and the transcription of the corresponding genes will be studied. The scientists will use transgenic plants and bacteria in which selected enzymes will be either silenced or overexpressed in order to study potential key roles of particular biosynthetic steps compared to those in untransformed organisms. The researchers also plan to transfer alternative or additional genes to the two bacterial species Escherichia coli and Clostridium ljungdahlii. Applied in high-volume fermenters, MEP pathway-optimized microorganisms should be able to produce higher quantities of the desired substances.
MEP pathway-optimized bacteria are also interesting for their use in chemical industries, especially in the context of processing so-called syngases. Syngas is a mixture of carbon dioxide, carbon monoxide and hydrogen that often accumulates in power plants and steel mills. MEP pathway-optimized Clostridium bacteria with a more efficient isoprene synthase could metabolize these three gases and form isoprene, which may be used to produce a special rubber. Alternatively, syngas could be an intermediate for the production of biofuels. Such a bacteria-based process would be superior to the conventional Fischer-Tropsch synthesis because unpurified syngas could be applied to the fermenters. The Fischer-Tropsch process, on the other hand, which is based on metal catalysis, requires highly purified syngas, and furthermore is highly energy consuming.
Drugs against malaria and cancer
The elucidation of the crucial regulatory steps of the MEP pathway may eventually contribute to the development and production of pharmaceuticals in human medicine, for example against cancer or malaria. The anti-cancer drug taxol and artemisinin, used for the treatment of malaria, could be produced by bacteria whose MEP pathway has been successfully modified.