The REACh Construct

On this page, we present our biosensor on the molecular level. Explore the different parts of our genetic device:

A Fluorescence Answer Faster Than Expression

Biosensors often work with a system that is comprised of a reported gene under the control of a promoter directly induced by the chemical that the sensor is supposed to detect. In the case of our 2D biosensor for Pseudomonas aeruginosa, the expression of our reporter gene, GFP, would be directly induced by the activity of the bacterium's quorum sensing molecules. However, transcription, translation, folding and post-translational modifications take their time. Since our goal is to detect the pathogen as fast as possible, we wanted to use a system that gives a fluorescent answer fast than just expressing the fluorescent protein.

Approach of a traditional biosensor In this model, the expression of GFP is directly controlled by a promoter whose activator binds to a molecule secreted by the pathogen.

Instead of the traditional approach, we constitutively express our reporter gene in a quenched form. As GFP-REACh fusion protein, fluorescence is suppressed. Our biosensor gives a response when homoserine lactones of Pseudomonas aeruginosa are taken up by our sensor cells where the autoinducer activates the expression of the TEV protease by binding to the LasI promoter in front of the protease gene.

Our novel biosensor approach Expression of the TEV protease is induced by HSL. The protease cleaves the GFP-REACh fusion protein to elicit a fluorescence response.

This approach has two advantages:

• When Pseudomonas aeruginosa is detected by our cells, the reporter protein has already been expressed and only waits to be cleaved off the REACh quencher. The cleavage reaction catalyzed by the TEV protease is a faster process than expression and correct folding of GFP. This way we hope for an earlier response by our sensor cells.

• While a certain concentration of homoserine lactone will produce the same number of gene read-outs, one TEV protease can cleave many GFP-REACh constructs. Through the cleavage step, we introduce an amplification step into our system. With the TEV protease, we will be able to produce a much stronger signal in a short time interval.

The FRET (Förster Resonance Energy Transfer) System

Förster resonance energy transfer (FRET), sometimes also called fluorescence resonance energy transfer, is a physical process of energy transfer. In FRET, the energy of a donor chromophore, whose electrons are in an excited state, is passed to a second chromophore, the acceptor. The energy is transferred without radiation and is therefore not exchanged via emission and absorption of photons. The acceptor then releases the energy received from the donor, for example, as light of a longer wavelength.

In biochemistry and cell biology, fluorescent dyes, which interact via FRET, are applied as "optical nano metering rules", because the intensity of the transfer is dependent on the spacing between donor and acceptor, and can be observed over distances of up to 10 nm. This way, protein-protein interactions and conformational changes of a variety of tagged biomolecules can be observed. For efficient FRET to occur, there must be a substantial overlap between the donor fluorescence emission spectrum and the acceptor fluorescence excitation (or absorption) spectrum ([http://www.nature.com/nprot/journal/v8/n2/full/nprot.2012.147.html Broussard et al., 2013]), as described in the figure below.

FRET betweeen donor and acceptor On the left: Jablonski diagram showing the transfer of energy between donor and acceptor; on the right: For successful FRET, the emission spectrum of of the donor has to overlap with the absorption spectrum of the acceptor.

However, fluorescence is not an essential requirement for FRET. This type of energy transfer can also be observed between donors that are capable of other forms of radiation, such as phosphorescence, bioluminescence or chemiluminescence, and fit acceptors. Acceptor chromophores do not necessarily emit the energy in form of light, and can lead to quenching instead. Thus, this kind of acceptors are also referred to as dark quenchers. In our project, we use a FRET system with a dark quencher, namely our REACh construct.

REACh Proteins - Dark Quenchers of GFP

In 2006, [http://www.pnas.org/content/103/11/4089.full Ganesan et al.] were the first to present a previously undescribed FRET acceptor, a non-fluorescent yellow fluorescent protein (YFP) mutant called REACh (for Resonance Energy-Accepting Chromoprotein). YFP can be used as a FRET acceptor in combination with GFP as the donor in FRET microscopy and miscellaneous assays in molecular biology. The ideal FRET couple should possess a large spectral overlap between donor emission and acceptor absorption - as illustrated in previous section - but have separated emission spectra to allow their selective imaging.

To optimize the spectral overlap of this FRET pair, the group obtained a genetically modified YFP acceptor. Mutations of amino acid residues that stabilize the excited state of the chromophore in enhanced YFP (EYFP) resulted in a non-fluorescent chromoprotein. Two mutations, H148V and Y145W, reduced the fluorescence emission by 82 and 98%, respectively. Ganesan et al. chose the Y145W mutant and the Y145W/H148V double mutant as FRET acceptors, and named them REACh1 and REACh2, respectively. Both REACh1 and REACh2 act as dark quenchers of GFP.

Producing a GFP-REACh Fusion Protein

In our project, we reproduced the REACh1 and REACh2 proteins by subjecting an RFC-25 compatible version of the BioBrick [http://parts.igem.org/Part:BBa_E0030 E0030] (EYFP) to a QuikChange mutation, creating the BioBricks [http://parts.igem.org/Part:BBa_K1319001 K1319001] and [http://parts.igem.org/Part:BBa_K1319002 K1319002]. Subsequently, we fused each REACh protein with GFP (mut3b) which is available as BioBrick [http://parts.igem.org/Part:BBa_E0040 E0040]. The protein complex was linked via a protease cleavage site, [http://parts.igem.org/Part:BBa_K1319016 K1319016]. As constitutive promoter we use [http://parts.igem.org/Part:BBa_J23101 J232101]. When GFP is connected to either REACh quencher, GFP will absorb light but the energy will be transferred to REACh via FRET and is then emitted in the form of heat; the fluorescence is quenched. Our cells also constitutively express the [http://parts.igem.org/Part:BBa_C0179 LasR] activator. Together with the HSL molecules from P. aeruginosa, LasR binds to the [http://parts.igem.org/Part:BBa_J64010 LasI] promoter that controls the expression of the TEV protease, which we make available as [http://parts.igem.org/Part:BBa_K1319004 K1319004]. When the fusion protein is cleaved by the TEV protease, REACh will be separated from GFP. The latter will then be able to absorb and emit light as usual.

Composition of our biosensor For our biosensor, we use a mix of already available and self-constructed BioBricks.

The resulting fusion proteins were labelled as [http://parts.igem.org/Part:BBa_K1319013 K1319013] (GFP fused with REACh1) and [http://parts.igem.org/Part:BBa_K1319014 K1319014] (GFP fused with REACh 2) and the linker between the proteins is labelled as [http://parts.igem.org/Part:BBa_K1319013 K1319016].

Cutting the Fusion Protein with the TEV Protease

To cut the GFP-REACh fusion protein off, we chose Tobacco Etch Virus (TEV) protease, a highly sequence-specific cysteine protease, that is frequently used for the controlled cleavage of fusion proteins in vitro and in vivo. The native protease also contains an internal self-cleavage site. This site is slowly cleaved to inactivate the enzyme. The physiological reason for the self-cleavage is unknown, however, undesired for our use. Therefore, our team uses a variant of the native TEV protease containing the mutation S219V which results in an alteration of the cleavage site so that self-inactivation is diminished.

TEV protease with a bound peptide This picture shows the TEV protease with a peptide chain bound in the binding pocket ready to be cleaved. The bound peptide chain has the recognition sequence inside the binding pocket. It was rendered with POV-Ray.

Though quite popular in molecular biology, the TEV protease is not avaiable as a BioBrick yet. Hence, the Aachen team introduces a protease with anti-self cleavage mutation S219V and codon optimized for E. coli [http://parts.igem.org/Part:BBa_K1319004 to the Parts Registry this year.]

Achievements

Characterization of GFP-REACh1 and GFP-REACh2 in Combination with an IPTG-inducible TEV Protease

The characterization of the TEV protease and the REACh1 and REACh2 dark quenchers was performed by introducing both simultaneously into E. coli. The resulting double plasmid cells therefore contained [http://parts.igem.org/Part:BBa_K1319013 K1319013] (GFP-REACh1 fusion protein) or [http://parts.igem.org/Part:BBa_K1319014 K1319014] (GFP-REACh2 fusion protein) and [http://parts.igem.org/Part:BBa_K1319008 K1319008] (IPTG-inducible TEV protease). K1319013 and K1319014 were cloned into a pSB3K3 plasmid backbone and K1319008 into a pSB1C3 backbone, two standard plasmids with different ORIs allowing simultaneous use in one cell.

[http://parts.igem.org/Part:BBa_I20260 I20260] was used as a positive control. I20260 contains the same promoter ([http://parts.igem.org/Part:BBa_J23101 J23101]), the same RBS ([http://parts.igem.org/Part:BBa_B0032 B0032]) and the same version of GFP ([http://parts.igem.org/Part:BBa_E0040 E0040]) and is located on the same plasmid backbone, pSB3K3. Therefore, it is expected that when all fusion proteins are successfully cut by the TEV protease, the fluorescence level of the double plasmid constructs reaches the same level as the positive control of I20260. As a negative control, we used [http://parts.igem.org/Part:BBa_B0015 B0015], a coding sequence of a terminator which should not show any sign of fluorescence.

To better evaluate the fluorescence, we took the observed OD into account in order to achieve a fluorecence reading independent of the amount of cells present. This way, the reading represents of the amount of fluorescence per cell only. To characterize our REACh1/2 constructs in combination with the TEV protease, we carried out a growth experiment. We compared both of the REACh-TEV constructs, a constitutive expression of GFP (I20260) as positive control and B0015 as negative control. For each expression, we ran IPTG-induced and uninduced cultures in parallel. All measurements were done in a biological triplicate.

Comparison of K1319013 + K1319008, K1319014 + K1319008, I20260 (positive control) and B0015 (negative control) Fluorescence was normalized by dividing by the optical density.

The negative control B0015 did not exhibit any significant fluorescence, as expected. The positive control I20260 showed a steady increase in fluorescence for the first 5 hours. After that, the fluorescence stayed constant due to the end of the exponential growth phase. In this state, growth becomesstationary and the cells do not produce GFP anymore. As expected, the production is also independent of addition of IPTG.

Both double plasmid constructs K1319013 + K1319008 and K1319014 + K1319008 did not exhibit a strong fluorescence before induction with IPTG. In the uninduced state, the fluorescence stays low and only increases slightly over time. It is significantly weaker than the fluorescence reached by the induced constructs or the positive control but was higher than the negative control. This shows that the promoter system used is not completely shut down without induction but significantly weaker compared to the induced state. The basal level of fluorescence might partly be due to an imperfect dark quenching of GFP by the REACh1 or REACh2.

The induced double plasmid constructs exhibited a fast rise in fluorescence after induction. The signal strenght increased 10-fold over the uninduced constructs. K1319013 + K1319008 reached the the same level of fluorescence as I20260, indicating a complete cleavage of the fusion proteins by the TEV protease. K1319014 + K1319008 did not reach a level of fluorescence as high as K1319013 + K1319008, however, the nearly 10-fold increase in fluorescence after induction is a clear indicator for the TEV protease cutting the fusion protein K1319014. The weaker fluorescence was probably due to a lower expression level of K1319014 in the cells in general.

Summary

The double plasmid systems of K1319013 + K1319008 and K1319014 + K1319008 clearly demonstrate the quenching ability of the REACh1 and REACh2 proteins as well as the funcionality of the TEV protease. Both REACH1 and REACH2 show a significant quenching ability of GFP shown in the difference of fluorescence between the positive control I20260 and the uninduced double plasmid systems. This has also been comfirmed by the sharp increase in fluorescence after induction showing that the TEV protease is successfully able to cut the fusion proteins, and proves a proper expression of both fusion proteins. Combined, this characterization is a validation of the functionality of the REACh1 protein ([http://parts.igem.org/Part:BBa_K1319001 K1319001]), the REACh2 protein ([http://parts.igem.org/Part:BBa_K1319002 K1319002]) and the TEV protease ([http://parts.igem.org/Part:BBa_K1319004 K1319004]).

Comparing Kinetics of the GFP-REACh fusion proteins with a Standard lacI-inducible GFP Expression

To assess the kinetics of the fusion proteins K1319013 (GFP-REACh1) and K1319014 (GFP-REACh2), the double plasmid systems K1319013 + K1319008 and K1319014 + K1319008 were compared to a standard expression of GFP under the control of a lacI promoter in [http://parts.igem.org/Part:BBa_K731520 K731520], a BioBrick made by the iGEM Team TRENTO in 2012. This way, we investigated the prediction a faster fluorescence response with our construct compared to a normal expression.

K731520 and the double plasmid constructs K1319013 + K1319008 and K1319014 + K1319008 were cultivated in E. coli BL21(DE3), and fluorescence and OD were measured. Once again, the fluorescence was adjusted for the OD to show a relative fluorescence on a per cell basis. Besides, we carefully compared the difference between the induced and uninduced state. This difference, the fluorescence quotient, serves as a better indicator for a system which is used as a sensor because the difference between an on and off state is more important for a clear and unmistakable signal than to the overall fluorescence. Hence, the OD-adjusted fluorescence quotient for both double plasmid constructs and K731520 was obtained and plotted in the following graph.

Fluorescence was normalized by dividing by the optical density. The fluorescence of induced cells was additionally divided by the fluorescence of uninduced cells to obtain the fluorescence quotient.

The graph clearly shows the faster response of the cut GFP-REACh fusion protein compared to a standard GFP expression. Both fluorescence signals of the double plasmid constructs achieve a higher difference in fluorescence signal between induced and uninduced state as well as at a faster rate. This proves the hypothesis made earlier about the kinetics of the GFP-REACh fusion protein combined with the TEV protease.

Summary

The kinetics of the fusion protein combined with the TEV protease exhibits the exact characteristics as predicted. The response is clearly faster than normal expression by accumulating a reservoir of fusion proteins which are not fluorescing due to the dark quencher attached to them. This reservoir is then activated by the induction of the TEV protease expression. Production of the protease results in the cleavage of the fusion protein, releasing GFP from the dark quencher and disturbing the interaction between the FRET pair. This results in the observed faster fluorescence reaction due to the amplificating effect of the TEV protease in which every one TEV protease can account for many fluorescence proteins being activated.

Characterizing the GFP-REACh Constructs in our Sensor Chips

K1319013 + K1319008 and K1319014 + K1319008 uninduced (top) and induced (bottom) in sensor cells The induced double plasmid systems K1319013 + K1319008 and K1319014 + K1319008 exhibit a clear fluorescence response in our sensor cells which in response to induction with 2 µl IPTG.

To further characterize the REACh construct, they were introduced into the sensor cells which were then induced with 2 µL IPTG with a concentration of 100 mM. Subsequently, we took fluorescence measurement read-outs (GFP, excitation 496 ± 9 nm, emission 516 ± 9 nm) roughly every 10 min in the plate reader. The results were plotted in the heatmap shown on the left.

The heatmap shows an increase of fluorescence from blue (no fluorescence) to red (high fluorescence). It is clearly visible that the induced chips are exhibiting a significantly higher fluorescence than the uninduced chips. This again shows that the constructs work as intended: The TEV protease cuts the linker so that the fusion protein is separated into GFP and a dark quencher, disabling the quenching. GFP has a clear fluorescence emission after the fusion protein has been successfully cut into two pieces by the TEV protease.

Outlook

The system of the GFP-REACh fusion proteins with an inducible TEV protease has been established and shows clearly the desired results of being faster than normal expression. The next step will be to engineer the TEV protease to be inducible by the HSL instead of IPTG and then to incorporate both the inducible TEV protease and the fusion protein on one plasmid backbone. This would also allow us to choose a high copy plasmid for both inserts, instead of a high copy plasmid for the TEV protease and a low to mid copy plasmid for the fusion protein which should yield an overall higher fluorescence readout.

Afterwards we would then like to characterize this construct the same way we have done with the double plasmid system and explore option for other fluorescence proteins than GFP to incorporate into our system with different quenchers to be able to have multiple fluorescence responses readable at the same time while being fast than the normal expression.

Also finding and testing different promoters to induce the TEV protease is planned to be able to detect not only Pseudomonas aeruginosa but also other pathogens or other relevant molecules in general so that we can establish a concept for faster recognition for a variety of uses.

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• Broussard, J. A., Rappaz, B., Webb, D. J., & Brown, C. M. (2013). Fluorescence resonance energy transfer microscopy as demonstrated by measuring the activation of the serine/threonine kinase Akt. Nature protocols, 8(2), 265-281. doi: 10.1073/pnas.0509922103

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