Part:BBa_K1127008
Gold sensing device (PgolTS+LacZ+GolS)
This device is constructed to recognize gold (HAuCl4) in its environment. GolS is a transcriptional activator of the promoter P(golTS). Upon binding with the gold, it dimerizes and binds to the recognition site within the promoter. We also include LacZ alpha that is useful for quantification of the device activity via Beta-galactosidase assay. The mechanism is simplified and shown in Figure 1.
The promoter PgolTS has identical function to the part BBa_K310009 submitted by the team iGEM 2010 UIUC-Illinois [1]. However, our promoter is much cleaner as it does not contain non-promoter sequences which may interfere with transcription of the genes downstream. In addition, codon usage of our golS has been optimised specifically for E. coli to ensure efficient translation.
Introduction
Gold ions are toxic to most of the organisms and can cause serious environmental problems. The toxicity appears from interactions of gold ions with proteins and later on with the membrane causing the cells to lyse (1). Some of the microorganisms developed proteins that are sensitive to certain metal ions and can activated the expression of responsive elements. For example CueR from E. coli can bind to gold ions as well as copper ions (2). Due to lack of specificity this promoter is not very useful in synthetic biology. We decided to use part of the golT/S operon from Salmonella typhimurium, except for golT protein. This operon is regulated only by gold ions (3) therefore we took the promoter PgolT/S and transcription activator golS.
Characterization
To determine activity of the gold sensing device, beta-galactosidase assays were run in triplicates using PNPG as the substrate. In all experiments, untransformed E. coli strain DH5 alpha or strain BL21 were used as our negative control. The protocols are described in more detail in the Protocols section.
We demonstrated that E. coli strain DH5 alpha transformed with BBa_K1127008 can respond to the gold. In Figure 2a, relative activity of the enzyme beta-galactosidase increased significantly in the presence of golS and promoter P(golTS) (DH5 alpha_AB) but reduced to the basal level when golS was missing (DH5 alpha_A). The results are well supported by statistics - Kruskal-Wallis X^2= 20.75, df = 7, p-value < 0.01. This signifies the function of golS as a gold-dependent transcriptional activator.
In Figure 2b, E. coli strain BL21 has background expression of the enzyme. Strong enzyme activities were observed in the absence of lacZ alpha (BL21 and BL21_B), so that no differences were observed (Kruskal-Wallis X^2= 6.69, df = 3, p-value > 0.05). This suggests that there could be beta-galactosidase in the chromosome of this E. coli strain. Further research has confirmed our hypothesis. Beta-galactosidase (accession no. = C6ELN6) is present in BL21 according to the gene database on uniprot.org.
We determined kinetics of the device. DH5 alpha were exposed to different gold concentrations for 16 hours to reach steady states. We found that the response was non-linear with distinct phases - exponential phase (0.0uM - 2.5uM AuCl4), stationary phase (2.5uM - 10.0uM AuCl4) and logarithmic decline phase (not shown). The data were plotted in Figure3a with simulation of the fitted logistic model. Please visit our Modelling section for more detail. As seen in the plot, the device is very sensitive to the gold as low as 0.5uM. Later on, it became saturated from about 10uM onwards. The saturation could be caused by several factor, such as maximum promoter activity, expression and stability of the proteins and diffusion limit of the gold. In Figure 3b, we ran more assays on the subpart P(golTS)+LacZ alpha to confirm the function of golS. It is obvious that the promoter alone can't respond to the gold (Kruskal-Wallis X^2= 4.68, df = 6, p-value > 0.5).
E. coli strain DH5 alpha were transformed with the device or one of the subparts. They were streaked on the LB agar plates supplemented with chloramphenicol and Xgal. If LacZ alpha is expressed, the enzyme will degrade the substrate Xgal and turn colonies blue. In Figure 4, the colonies appear blue apart from those on the DH5 alpha+golS plate. This supports our hypothesis that P(golTS) is the minimal unit for the expression of LacZ alpha.
References
1. Goodman, C. M., McCusker, C. D., Yilmaz, T. & Rotello, V. M. Toxicity of Gold Nanoparticles Functionalized with Cationic and Anionic Side Chains. Bioconjugate Chem. 15, 897-900 (2004).
2. Stoyanpv, J. V. & Brown, N. L. The Escherichia coli copper-responsive copA promoter is activated by gold. J. Biol. Chem. 273, 1407-1410 (2003).
3. Wei, W. et al Engineering a gold-specific regulon for cell-based visual detection and recovery of gold. Chem. Sci. 3, 1780-1784 (2012). Sequence and Features
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