Rational design of an improved induction scheme for recombinant Escherichia coli

Engineering of genetic control tools in Synechocystis sp. PCC 6803 using rational design techniques

Cyanobacteria show promise as photosynthetic microbial factories capable of harnessing sunlight and CO2 to produce valuable end products, but few genetic control tools have been characterized and utilized in these organisms. To develop a suite of control elements capable of gene control at a variety of expression strengths, a library of 10 promoter-constructs were developed and built via rational design techniques by adding individual nucleotides in a step-wise manner within the -10 and -35 cis-acting regions of the tac promoter. This suite produced a dynamic range of expression strength, exhibiting a 78 fold change between the lowest expressing promoter, Psca8- and the highest expressing promoter, Psca3-2 when tested within Synechocystis sp. PCC 6803. Additionally, this study details the construction of a chemically inducible construct for use in Synechocystis that is based on the tac repressor system most commonly used in Escherichia coli. This research demonstrates the construction of a highly expressed inducible promoter that is also capable of high levels of gene repression. Upon chemical induction with IPTG, this same mutant strain was capable of exhibiting an average 24X increase in GFP expression over that of the repressed state.

Galactose can be an inducer for production of therapeutic proteins by auto-induction using E. coli BL21 strains

Recently lactose mediated auto-induction in Escherichia coli has gained a lot of interest because higher protein titer could be achieved without the need to monitor growth and add inducer at the proper time. In this study a high level therapeutic protein production by auto-induction was observed in E. coli BL21 using either T7 or tac promoters in the modified Luria Bertani (mLB) medium containing soy peptone instead of tryptone in Luria Bertani (LB) medium. Based on medium analysis and spiking experiments it was found that 0.4 mM galactose from the soy peptone caused the auto-induction. E. coli cultures induced by galactose can saturate at considerably higher density than cultures induced by IPTG. Galactose is not consumed by E. coli BL21. Finally it has been demonstrated that auto-induction can be effectively used in fed-batch fermentation for the industrial production of a therapeutic protein. The principle of galactose mediated auto-induction should be able to apply to high throughput microplates, shake flasks and fed-batch fermentors for clone screening and therapeutic protein expression in E. coli gal(-) strains such as most commonly used BL21.

Productivity improvement of recombinant Escherichia coli fermentation via robust optimization

A nonlinear model of a recombinant Escherichia coli producing porcine growth hormone (pGH) fermentation was developed. The model was used to calculate a glucose feeding and temperature strategy to optimize the production of pGH. Simulations showed that the implementation of optimal feed and temperature profiles was sensitive to the maximum specific growth rate, and a mismatch could result in excessive acetate production and a significant reduction in pGH yield. An optimization algorithm was thus developed, using feedback control, to counter the effects of uncertainty in the specific growth rate and thus determine an optimal operating strategy for pGH production. This policy was experimentally implemented in a 10 L fermenter and resulted in a 125% increase in productivity over the previous best experimental result with this system--in spite of significant plant-model mismatch.

Effect of Inducers on the Production of 5‐Aminolevulinic Acid by RecombinantEscherichia coli

Aminolevulinic acid (ALA) was produced by recombinant Escherichia coli BL21(DE3) (pET28-A.R-hemA) harboring the ALA synthase gene (hemA) from Agrobacterium radiobacter zju-0121. The effects of inducers on the ALA synthase activity and ALA productivity were evaluated. The results indicated that a low isopropyl-beta-D-thiogalactoside (IPTG) concentration (0.05 mmol/L) was favorable for high expression of ALA synthase, which resulted in higher ALA productivity. For metabolic engineering applications, lactose was a better substitute of IPTG for active enzyme expression. When lactose concentration was 5 mmol/L, the specific ALA synthase activity and ALA productivity reached 16.7 nmol/(min . mg of protein) and 1.15 g/L, respectively, which were about 15% and 43% higher than those induced by IPTG.

Metabolic engineering of Escherichia coli for industrial production of glucosamine and N-acetylglucosamine

Glucosamine and N-acetylglucosamine are currently produced by extraction and acid hydrolysis of chitin from shellfish waste. Production could be limited by the amount of raw material available and the product potentially carries the risk of shellfish protein contamination. Escherichia coli was modified by metabolic engineering to develop a fermentation process. Over-expression of glucosamine synthase (GlmS) and inactivation of catabolic genes increased glucosamine production by 15 fold, reaching 60 mg l(-1). Since GlmS is strongly inhibited by glucosamine-6-P, GlmS variants were generated via error-prone PCR and screened. Over-expression of an improved enzyme led to a glucosamine titer of 17 g l(-1). Rapid degradation of glucosamine and inhibitory effects of glucosamine and its degradation products on host cells limited further improvement. An alternative fermentation product, N-acetylglucosamine, is stable, non-inhibitory to the host and readily hydrolyzed to glucosamine under acidic conditions. Therefore, the glucosamine pathway was extended to N-acetylglucosamine by over-expressing a heterologous glucosamine-6-P N-acetyltransferase. Using a simple and low-cost fermentation process developed for this strain, over 110 g l(-1) of N-acetylglucosamine was produced.