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key metabolic pathway for construction of an inducer-free L-homoserine-producing strain to maximize the productivity of L-homoserine based on genetic-engineering tools, comparison of L-homoserine production, cell growth, and glucose consumption in different engineered strains, overview. L-Homoserine is a nonessential amino acid for the biosynthesis of L-threonine and L-methionine. It is also an important precursor for the production of isobutanol, 1,4-butanediol, L-phosphinothricin, 2,4-ihydroxybutyrate, and 1,3-propanediol. The initial L-homoserine-producing strain HS1 is obtained by blocking the degradative and competitive pathways and overexpressing thrA (encoding homoserine dehydrogenase) based on an O-succinyl homoserine-producing strain, using the pull-push-block strategy, an efficient method to engineer microorganisms involved in biosynthesizing target products by modifying metabolic networks. L-homoserine-converting pathway-related genes (thrB, encoding homoserine kinase, and metA, encoding homoserine O-succinyltransferase) are successively deleted to block L-homoserine degradation. Gene thrA is overexpressed to push the carbon flux to L-homoserine production. Then, the lysine-auxotrophic strain HS2 is generated by deleting lysA to eliminate a precursor competing metabolic pathway on L-homoserine production. For strengthening the capability of the L-homoserine transport system and the transformation of other toxic intermediate metabolites, gene rhtA, encoding the inner membrane transporter that is involved in the export of L-homoserine, is overexpressed chromosomally by replacing the native promoter with the trc promoter to obtain strain HS3 (Trc-rhtA). The strain shows increased activity. Increase in the L-homoserine export capacity and relieve the growth burden of homoserine-producing strains to enable survival via replacement of the native promoter of the eamA gene by the trc promoter in strain HS4 (Trc-eamA). Two rhtA gene copies (the native rhtA gene and replacement of the lacI gene) and eamA are overexpressed under the trc promoter in the chromosome to construct strain HS5 (DELTAlacI::Trc-rhtA Trc-rhtA Trc-eamA). Under batch culture, strain HS5, with modification of the transport system and construction of a constitutive expression system, can produce 3.14 g/l L-homoserine, which is 54.2% higher than strain HS2 production. In addition, the specific production of strain HS5 is also increased. Repression of candidate genes by the CRISPRi system to further enhance L-homoserine production |
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