ABSTRACT
The introduction of the mevalonate pathway (MVA pathway) in recombinant Escherichia coli can improve the synthesis of terpenoids. But the imbalance expression of MVA pathway genes and accumulation of intermediates inhibit cell growth and terpenoids production. In this study, each gene of MVA pathway and key genes of lycopene synthesis pathway were cloned in plasmid to express in the recombinant E. coli LYC103 with optimizing the expression of the key genes of the 2-methyl-D-erythritol-4-phosphate pathway (MEP pathway), chromosome recombinant MVA pathway and the lycopene synthesis pathway. The results showed that the overexpression of ispA, crtE, mvaK1, idi and mvaD genes did not affect the cell growth, while lycopene production increased by 13.5%, 16.5%, 17.95%, 33.7% and 61.1% respectively, indicating that these genes may be the rate-limiting steps for the synthesis of lycopene. mvaK1, mvaK2, mvaD of MVA pathway were the rate-limiting steps and were in an operon. The mvaK1, mvaK2, mvaD operon was regulated by mRS (mRNA stabilizing region) library in front of mvaK1, obtaining strain LYC104. Lycopene yield of LYC104 was doubled and cell growth was increased by 32% compared with the control strain LYC103. CRISPR-cas9 technology was used to integrate idi into chromosome at lacZ site to obtain LYC105 strain. Cell growth of LYC105 was increased by 147% and lycopene yield was increased by 2.28 times compared with that of LYC103. In this study, each gene of lycopene synthesis pathway was expressed in plasmid to certify the rate-limiting gene based on the complete MVA pathway on the chromosome. Then the rate-limiting gene was integrated in chromosome with homologous recombination to release the rate-limiting, which providing a new strategy for the construction of high-yield strains for metabolic engineering.
ABSTRACT
Isoprenoids are all derived from two five-carbon building blocks called isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), which are synthesized either by the mevalonate (MVA) pathway or 2-C-methyld-D-erythritol-4-phosphate (MEP) pathway. In this study, the MVA pathway genes were integrated into the chromosome of LYC101, in which the expression of key genes in the MEP synthesis pathway and lycopene synthesis pathway were optimized by artificial regulatory parts, to further improve the production of isoprenoids in Escherichia coli. The plasmids pALV23 and pALV145 were screened from a plasmid library that constructed by using the RBS library to link the genes of the MVA pathway, which greatly increased the production of β-carotene. The effects of plasmids pALV23 and pALV145 on the lycopene production in low and high lycopene production strain, LYC001 and LYC101, were compared, respectively. The production of lycopene was increased by plasmids pALV23 and pALV145 in both strains. In high lycopene production strain LYC101, pALV23 produced more lycopene than pALV145. Then, the MVA gene together of promoter of pALV23 was integrated into the chromosome of LYC101 at poxB site using method of homologous recombination helped by CRISPR-Cas9 system, resulted in genetically stable strain, LYC102. The yield of lycopene of LYC102 was 40.9 mg/g DCW, 1.19-folds higher than that of LYC101, and 20% more than that of LYC101 with pALV23. Simultaneous expression of MVA pathway and MEP pathway in recombinant E. coli can effectively increase the yield of terpenoids. In this study, a plasmid-free, genetically stable, high-yielding lycopene strain was constructed, which could be used for industrialization. Also, the platform strain can be used for the synthesis of other terpenoids.