Characterization of salt-tolerant mutant for enhancement of L-threonine production in Escherichia coli

Combined metabolic analyses for the biosynthesis pathway of l-threonine in Escherichia coli

Currently, industrial production of l-threonine (Thr) is based on direct fermentation with microorganisms such as Escherichia coli, which has the characteristics of low cost and high productivity. In order to elucidate the key metabolic features of the synthesis pathway of Thr in E. coli to provide clues for metabolic regulation or engineering of the strain, this study was carried out on an l-threonine over-producing strain, in terms of analyses of metabolic flux, enzyme control and metabonomics. Since environmental disturbance and genetic modification are considered to be two important methods of metabolic analysis, addition of phosphate in the media and comparison of strains with different genotypes were selected as the two candidates due to their significant influence in the biosynthesis of Thr. Some important targets including key nodes, enzymes and biomarkers were identified, which may provide target sites for rational design through engineering the Thrproducing strain. Finally, metabolic regulation aimed at one biomarker identified in this study was set as an example, which confirms that combined metabolic analyses may guide to improve the production of threonine in E. coli.

Global analyses of transcriptomes and proteomes of a parent strain and an L-threonine-overproducing mutant strain

We compared the transcriptome, proteome, and nucleotide sequences between the parent strain Escherichia coli W3110 and the L-threonine-overproducing mutant E. coli TF5015. DNA macroarrays were used to measure mRNA levels for all of the genes of E. coli, and two-dimensional gel electrophoresis was used to compare protein levels. It was observed that only 54 of 4,290 genes (1.3%) exhibited differential expression profiles. Typically, genes such as aceA, aceB, icdA, gltA, glnA, leu operon, proA, thrA, thrC, and yigJ, which are involved in the glyoxylate shunt, the tricarboxylic acid cycle, and amino acid biosynthesis (L-glutamine, L-leucine, proline, and L-threonine), were significantly upregulated, whereas the genes dadAX, hdeA, hdeB, ompF, oppA, oppB, oppF, yfiD, and many ribosomal protein genes were downregulated in TF5015 compared to W3110. The differential expression such as upregulation of thr operon and expression of yigJ would result in an accumulation of L-threonine in TF5015. Furthermore, two significant mutations, thrA345 and ilvA97, which are essential for overproduction of L-threonine, were identified in TF5015 by the sequence analysis. In particular, expression of the mutated thrABC (pATF92) in W3110 resulted in a significant incremental effect on L-threonine production. Upregulation of aceBA and downregulation of b1795, hdeAB, oppA, and yfiD seem to be linked to a low accumulation of acetate in TF5015. Such comprehensive analyses provide information regarding the regulatory mechanism of L-threonine production and the physiological consequences in the mutant stain.