Metabolic engineering of Escherichia coli and Corynebacterium glutamicum for the production of l-threonine

l-threonine is an essential amino acid for mammals and as such has a wide and expanding application in industry with a fast growing market demand. The major method of production of l-threonine is microbial fermentation. To optimize l-threonine production the fundamental solution is to develop robust microbial strains with high productivity and stability. Metabolic engineering provides an effective alternative to the random mutation for strain development. In this review, the updated information on genetics and molecular mechanisms for regulation of l-threonine pathways in Escherichia coli and Corynebacterium glutamicum are summarized, including l-threonine biosynthesis, intracellular consumption and trans-membrane export. Upon such knowledge, genetically defined l-threonine producing strains have been successfully constructed, some of which have already achieved the productivity of industrial producing strains. Furthermore, strategies for strain construction are proposed and potential problems are identified and discussed. Finally, the outlook for future strategies to construct industrially advantageous strains with respect to recent advances in biology has been considered.