Increased Threonine in Bacteria,Metabolic Engineering,Brevibacterium Lactofermentum,Aspartokinase,Lysine,Homoserine Dehydrogenase,Homoserine Kinase

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Metabolic Engineering and Metabolomics Metabolic Engineering and Metabolomics Introduction Cloning and Expression of Heterologous Genes Completion of Partial Pathways giving new Products Transfer of an Entire Biosynthetic Pathway Creating new Products and new Reactants Altering Nutrient uptake and Metabolite Transfer of Promising Natural Motifs Redirecting Metabolite Flow Desensitizing Feedback Inhibition Increased Threonine in Bacteria Increased Lysine Content in Crop Plants Elevation of the Activity of rate Limiting Enzymes Alteration in Protein Processing Pathway Reduction of Competition for Limited Resources Modification of Metabolic Regulation Molecular Breeding of Biosynthetic Pathways

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Increased threonine in bacteria
One of the popular examples involving desensitizing feedback inhibition for matabolic engineering includes improved threonine production by Brevibacterium lactofermentum. A mutant M-15 was isolated, which lacked feedback inhibition of aspartokinase by threonine and lysine, and of homoserine dehydrogenase (HD) by threonine. When cloned HD and HK (homoserine kinase) genes were transferred and their expression amplified in this mutant, the production of threonine increased from 17 .5g/liter to 33g/liter. In the mutant, aspartokinase and HD could not be rendered inactive by lysine and threonine respectively; the amplified HD and HK expression redirected ASA (aspartate semialdehyde) towards synthesis of threonine rather than lysine.

Threonine production by M-15 could be increased by yet another method. Expression of coned phosphoenol- pyruvate carboxylase (PEP II case) in M-15 led to 15% increase in threonine production, due to increased flow towards oxaloacetate (OAA) rather than towards acetyl CoA. In another lysine producing strain, transfer of gene for a mutant HD enzyme (not responding to feedback inhibition) led to shift of lysine concentration from 65glliter to 4g/1iter and that of threonine from zero to 52g/liter.

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