Design

Overview

In our project, we plan to realize the efficiently production of tryptophan in E.coli. Based on the biosynthesis pathway of tryptophan, aroG(encoding 3-deoxy-7-phosphoheptulonate synthase) and trpBA (encoding tryptophan synthase) were used to improve the yield of tryptophan (Fig.1.1). aroG can divert the intermediate products of glycolysis into the chorismate synthesis pathway, while trpBA can synthesize the precursor chorismate into tryptophan.[1-4])

We consider that excessive tryptophan production could interfere the proliferation of E.coli, for over-expressed aroG will competitive inhibit the glycolysis pathway while large amount of ATP and NADPH are consumed during the synthesis. Therefore, a toggle-switch circuit are used to reconciling the contradiction between cell proliferation and tryptophan production. In one of the bistable state, over expression of pykA(encoding pyruvate kinase II) is used to eliminate the competitive inhibition of aroG, reducing the production process and enabling a rapid cell proliferation. When cells reach a high density, they can be induced into another state where aroG and trpBA are expressed to product tryptophan efficiently (Fig.1.2).

Here two types of inducible promoter, Pλ promoter and lacUV5 promoter ,are used, which can be induced by heat (>42℃) and IPTG respectively. When lacUV5 promoter activated by IPTG, the cells will enter the "proliferation" state; while Pλ promoter is activated by heat, they will turn into the "production" state for expression of aroG and trpBA.

In order to test whether each part are functional individually, our working circuit is divided into several devives.

In order to realize the family application of our project and the automatic control of production conditions, we have designed a cultivation device. It contains controlling, detecting and cultivating modules (Fig.1.3). The equipment can monitor the cell growth and tryptophan production while conducting fermentation culture, and control the induction culture conditions through singlechip at different stages with this signal, so as to activate the expression of specific genes in the gene circuit, controlling the cells into "proliferation/production" state.

Considering it is difficult to realize real-time detection of tryptophan concentration by chemical method in hardware, we also have designed a detecting circuit which can sense the concentration of tryptophan and report green fluorescence of different light intensities (inversely proportional to the concentration of tryptophan) (Fig.1.4). By introducing this circuit into the engineered bacteria, the concentration of tryptophan in culture medium can be converted into light intensity that can be detected by hardware.

Through the interaction of hardware circuit and gene circuit, we can achieve both the automatic control and the high production, giving a full play to the advantages and potential of Synthetic Biology.

Reference

[1] SHEN T，LIU Q，XIE X，et al． Improved production of tryptophan in genetically engineered Escherichia coli with TktA and PpsA overexpression[J]．J Biomed Biotechnol，2012 (11) : 605219．
[2] CHEN L，ZENG A P．Rational design and metabolic analysis of Escherichia coli for effective production of L -tryptophan at high concentration[J]. Applied Microbiology and Biotechnology，2017，101( 2) : 559－568．
[3] Zhan JJ,Du LH. Progress of metabolic engineering modification of Escherichia coli for L-tryptophan production[J]. Shandong Chemical Industry,2021,50(01):85-87+89.
[4] Hu,Changyun.Study on the structure and function of 3-deoxy-D-arabinoheptulose-7-phosphate synthase AroG[D]. Fudan University, 2003.
[5] HAO Dali et al. Site-mutation of AroG Gene and Co-expression with TrpBA Gene in Escherichia coli. Chinese Journal of Applied and Environmental Biology 19, 817-821 (2013).

Experiment Result

Until now, we have successfully constructed aroG test circuit(BB..), AroG-trpAB synthetic circuit? pykA test circuit? Toggle switch with GFP/RFP.

And we have demonstrated that, the overexpression of aroG-S211F mutant has significantly improved the tryptophan production with a highest productivity of 160 mg/ml per OD. The detailed structural mechanism of the improvement has also been elucidated by modeling.

In addition, the toggle-switch circuit with GFP/RFP was successfully constructed, and demonstrated to be functional by RT-qPCR and fluorescence measurement. The circuit achieved the bistable expression of sfGFP and mRFP under different induction conditions, and can be fast switched by the change of inducers.

Verification of aroG-S211F (BBa_K3832000) ——Functional to improve the tryptophan production

AroG located in the branching point of the glycolysis and shikimate pathways, and overexpression of AroG has also been reported to be able to increase the production of downstream product in shikimate pathway. In our project, we constructed its mutant, aroG-S211F (Part:BBa_K3832000) and demonstrated its positive effect on the improvement of tryptophan production.

Verification of toggle-switch circuit with GFP and GFP (BBa_K3832007) —Functional to achieve the separated bistability and fast switch by two inducers.

In this stage, GFP and RFP are used to help us detect and check whether our design of toggle-switch circuit will work properly (Fig.2.4). Each promoter downstream follows another promoter's repressor gene and a fluorescent protein in different color. By detecting the relative fluorescence intensity under different inducing conditions, we can get information about the intensity of each promoter in different states and whether they can function as we designed.

Our experiment result have confirmed the feasibility of our design. As shown in Fig.2.5, we successfully achieved bistable protein expression under different induction conditions. The relative expression levels of GFP and RFP were significantly reversed with the change of inducting conditions, indicating that the circuit design can be used to switch cell states between “proliferation” and “production”

More details for the experiment design and result of toggle-switch circuit:
Engineering Success: toggle-switch circuit

Conclusion

The results of function of aroG and toggle-Switch circuits confirmed the feasibility of our general design. The expression of aroG-S211F can significantly promote the production of tryptophan and observed its inhibitory effect on cell proliferation. Meanwhile, for the control circuit, two groups of promoter-repressor systems and RBS with corresponding strength we used can achieve separated bistable state and fast switch under different inducers.

The results also confirmed our hypothesis that cell proliferation and tryptophan production should be separated, and it has been designed to be strictly controlled by toggle-switch circuit, in which cell proliferation (pykA gene overexpression) and tryptophan production (aroG-S211F overexpression) will be constructed in the two arms of toggle-switch. The one arm of toggle switch will be used for the expression of aroG-S211F and trpAB for tryptophan synthesis (production state), and another arm for the expression of pykA gene for cell growth (proliferation state), thus the cell growth and tryptophan synthesis can be will regulated and balanced.

Although the toggle-switch with aroG-S211F, trpBA and pykA genes for tryptophan synthesis was not verified experimentally, we confirmed its feasibility by modeling based on the data obtained in the above experiment (Fig.3.1 also see results on our Modelling page). Furthermore, we predict the best time to change the induction condition from IPTG to 42℃ heat when the tryptophan production reaches the maximum (about 1,170min), providing suitable parameters for automatic control of hardware and our further optimizations.