Increasing L-threonine production in Escherichia coli by overexpressing the gene cluster phaCAB.

L-Threonine is an important branched-chain amino acid and could be applied in feed, drugs, and food. In this study, L-threonine production in an L-threonine-producing Escherichia coli strain TWF001 was significantly increased by overexpressing the gene cluster phaCAB from Ralstonia eutropha. TWF001/pFW01-phaCAB could produce 96.4-g/L L-threonine in 3-L fermenter and 133.5-g/L L-threonine in 10-L fermenter, respectively. In addition, TWF001/pFW01-phaCAB produced 216% more acetyl-CoA, 43% more malate, and much less acetate than the vector control TWF001/pFW01, and meanwhile, TWF001/pFW01-phaCAB produced poly-3-hydroxybutyrate, while TWF001/pFW01 did not. Transcription analysis showed that the key genes in the L-threonine biosynthetic pathway were up-regulated, the genes relevant to the acetate formation were down-regulated, and the gene acs encoding the enzyme which converts acetate to acetyl-CoA was up-regulated. The results suggested that overexpression of the gene cluster phaCAB in E. coli benefits the enhancement of L-threonine production.