Improving L-threonine production in Escherichia coli by elimination of transporters ProP and ProVWX.

Background: Betaine, an osmoprotective compatible solute, has been used to improve L-threonine production in engineered Escherichia coli L-threonine producer. Betaine supplementation upregulates the expression of zwf encoding glucose-6-phosphate dehydrogenase, leading to the increase of NADPH, which is beneficial for L-threonine production. In E. coli, betaine can be taken through ProP encoded by proP or ProVWX encoded by proVWX. ProP is a H-osmolyte symporter, whereas ProVWX is an ABC transporter. ProP and ProVWX mediate osmotic stress protection by transporting zwitterionic osmolytes, including glycine betaine. Betaine can also be synthesized in E. coli by enzymes encoded by betABIT. However, the influence of ProP, ProVWX and betABIT on L-threonine production in E. coli has not been investigated.

Results: In this study, the influence of ProP, ProVWX and betABIT on L-threonine production in E. coli has been investigated. Addition of betaine slightly improved the growth of the L-threonine producing E. coli strain TWF001 as well as the L-threonine production. Deletion of betABIT retarded the growth of TWF001 and slightly decreased the L-threonine production. However, deletion of proP or/and proVWX significantly increased the L-threonine production. When proP was deleted, the L-threonine production increased 33.3%; when proVWX was deleted, the L-threonine production increased 40.0%. When both proP and proVWX were deleted, the resulting strain TSW003 produced 23.5 g/l L-threonine after 36 h flask cultivation. The genes betABIT, proC, fadR, crr and ptsG were individually deleted from TSW003, and it was found that further absence of either crr (TWS008) or ptsG (TWS009) improved L-threonine production. TSW008 produced 24.9 g/l L-threonine after 36 h flask cultivation with a yield of 0.62 g/g glucose and a productivity of 0.69 g/l/h. TSW009 produced 26 g/l L-threonine after 48 h flask cultivation with a yield of 0.65 g/g glucose and a productivity of 0.54 g/l/h, which is 116% increase compared to the control TWF001.

Conclusions: In this study, L-threonine-producing E. coli strains TSW008 and TSW009 with high L-threonine productivity were developed by regulating the intracellular osmotic pressure. This strategy could be used to improve the production of other products in microorganisms.