Construction And Optimization Of Escherichia Coli Cell Factories For β-carotene Production

β-carotene is a representative of carotenoid family,which shows strong ability of free radical avenging and singlet-oxygen quenching.It has been widely used in medicine,food,and cosmetic industires.Escherichia coli is an excellent host for microbial production of β-carotene,since it has clear genetic background,and is easy to be engineered and cultivated.In this study,E.coli cell factories for efficient β-carotene production were obtained by introducing β-carotene synthetic pathway into E.coli,followed by modulation of MEP and central metabolic modules directly in Escherichia coli chromosome.Firstly,eight isoprenoid genes of the MEP pathway were modulated with six artificial regulatory parts having a wide range of strengths to investigate their effects on β-carotene production.Optimal strength for each isoprenoid gene expression was identified,leading to 1.2 to 3.5-fold increase in β-carotene production.Our work suggested that modulating dxr, ispG and ispH genes with appropriate strengths could also increase β-carotene production.Secondly,in order to avoid plasmid instability,β-carotene synthetic gene operon was integrated into chromosome of E.coli ATCC8739.Modulating dxs and idi genes in combination resulted in 3.5-fold increase of P-carotene yield,while engineering β-carotene synthesis module through modulating crt gene operon resulted in another 3.4-fold increase.Thirdly,ATP and NADPH are two important cofactors for production of terpenoids compounds.Three central metabolic modules,including ATP synthesis,pentose phosphate and TCA,were further engineered to increase ATP and NADPH supplies.Modulation of our essential genes of ATP synthesis module caused up to 20% increase of β-carotene yield, while modulation of three essential genes of pentose phosphate module caused up to 17% increase.Modulation of nine genes of TCA module caused up to 39% increase of β-carotene yield.The better effect of engineering TCA module was suggested to be due to increased supplies of both ATP and NADPH,while engineering ATP synthesis and PPP modules would only increase supply of one cofactor.Combined engineering of TCA and PPP modules had a synergistic effect on improving β-carotene yield,while combined engineering of TCA and ATP synthesis modules did not.The best strain CAR005 was obtained by combined modulation of α-ketoglutarate dehydrogenase,succinate dehydrogenase and transaldolase B genes,which exhibited 64% increase of β-carotene yield over a high producing parental strain.In summary,through combined engineering of MEP,β-carotene synthesis and central metabolic modules,a genetically stable E.coli strain was obtained which exhibited 74-fold yield increase over the wild type.This strain produced 2.1 g/L β-carotene with a yield of 60 mg/g.To the best of our knowledge,this was the highest β-carotene titer and yield obtained for engineered E.coli.