Part:BBa_K554012:Experience

This part was used to [http://2011.igem.org/Team:UNICAMP-EMSE_Brazil/Results#Device_2_testing:_SoxR.2FSoxS_system_regulating_GFP_production Device 2 testing: SoxR/SoxS system regulated GFP production]

Experimental results

In order to test the ability of Jedi Bacteria in sensing NO levels and activating genes linked to SoxS promoter, we built a Device testing, with GFP linked to SoxS promoter, as it is shown in the following schema (Figure 1):

Figure 1: Testing [http://2011.igem.org/Team:UNICAMP-EMSE_Brazil/Project#Device_2:_NO_sensor.2FIL-10_producer Device 2] through replacement of IL-10 to GFP.

Methods

Competent E. coli DH5α strain cells were transformed with a pSB1A2 vector (Ampicillin resistant) carrying both the sensor (Strong_Constitutive_promoter + RBS + SoxR + Terminator) and the effector (SoxS_pormoter + RBS + GFP + HlyA + Terminator) devices using a chemical shock protocol. Transformed bacteria were plated on solid LB medium with 50 μg/mL Ampicillin and grown at 37ºC overnight. Surviving colonies were grown on liquid LB medium containing 50 μg/mL Ampicillin. Oxidative stress was induced by adding increasing concentrations of [http://en.wikipedia.org/wiki/Paraquat Paraquat] (Methyl viologen dichloride hydrate - Sigma), an oxidative stress inducer in bacteria. Final Paraquat concentrations were: 0 μM (control), 5 μM, 10 μM, 20 μM, 30 μM and 40 μM. Induction of the designed sensor/effector mechanism was accessed by fluorescence using a fluorometer (SLM – Aminco; 4 nm bandpass and 10 mm) with excitation in 500 nm and emission spectra from 508-550 nm.

In order to access if increasing concentrations of Paraquat could inhibit culture growth due to it´s toxicity, optical density (OD) levels of the culture were measured during the incubation time with Paraquat. The ability to recognize NO (nitric oxide), an inflammation signal molecule, was characterized for SoxR/SoxS sensor, and found to be FUNCTIONAL. In the section below we present the detailed results.

Results

Achieved results indicated that the designed sensor/effector device system was capable of inducible production of GFP (or another generic protein controlled by the sensor). In addition, protein induction can be modulated through varying inducer concentration (Figure 2). Higher concentrations of Paraquat exhibited higher fluorescence levels, which indicates increased GFP concentrations. No plateau was achieved using the highest tested concentrations.

Figure 2: SoxS/SoxR fluorescence data for concentrations 0, 5, 10, 20, 30 and 40 µM of inducer (Paraquat).

Moreover, we tested if Paraquat could be toxic for the bacteria cells, and we found that the experimental concentrations of Paraquat did not show significant differences in cell growth as shown by OD levels in Figure 3.

Figure 3: SoxS/SoxR optical density data for concentrations 0, 5, 10, 20, 30 and 40 µM of inducer (Paraquat).

Figure 4. Determination of specific growth rate (μ) for cells in the control (0 μM) and experimental (40 μM) conditions. X is the cellular concentration per volume. Specific growth rate (μ) is equal to the slope of the plot lnX x time, described by the equation lnX= lnX0 + μt.

Fluorescence Microscopy experiments also corroborates GFP production through Paraquat induction of SoxR/SoxS sensor/effector system. Fluorescence was detected in the cultures containing the sensor and GFP production devices when induced with 40 uM of Paraquat (Figure 5), but not in non-induced cultures(Figure 6). This is an additional evidence that the NO sensor system and Sox driven effector (in this case, GFP) synthesis worked as expected.

Figure 5: GFP fluorescence assessed by microscopy in 40 μM Paraquat induced culture. A) Fluorescence microscopy 40X Exp.: 0.478 ms; B) Light microscopy 40X; C) Fluorescence microscopy 100X Exp.: 0.478 ms; D) Light microscopy 100X.

Figure 6: GFP fluorescence assessed by microscopy in non-induced culture. A) Fluorescence microscopy 40X Exp.: 0.478 ms; B) Light microscopy 40X; C) Fluorescence microscopy 100X Exp.: 0.478 ms; D) Light microscopy 100X.

Discussion

The SoxR/SoxS system is one of the best characterized redox-sensing mechanisms in bacteria. Under oxidative stress conditions, activation of this system results in a cascade effect that ends in the activation of more than 16 genes that can counteract the harmful effect of superoxide and other oxidative radicals (Pomposiello and Demple 2001).

This device is a modification of Stanford team anti-inflammatory device (iGEM 2009 - http://2009.igem.org/Team:Stanford/ProjectPage), which comprises SoxR gene (BBa_K223047) under control of a Constitutive Promoter (BBa_J23119) and SoxS promoter (BBa_K223001 – deleted part). Although this system has already been designed and used in previously iGEM competitions (iGEM 2009 - http://2009.igem.org/Team:Stanford/ProjectPage), we took advantage of the availability of synthesized sequences to design new parts (SoxR and SoxS) that conforms the common biobrick standards in the iGEM registry. Our team has improved the parts as described in [http://2011.igem.org/Team:UNICAMP-EMSE_Brazil/Project#UNICAMP-EMSE_Brazil_Parts_Design:_why_are_they_different_to_Stanford_2009_team_ones.3F Innovation].

These newly designed parts were capable of sensing different concentrations of the inducer, resulting in an increased production of the protein under control of the SoxS promoter (in this case, GFP).

Under the experimental conditions, no cell growth inhibition was observed and confirmed by the calculations of the specific growth rate (μ). No significant difference was found in control experiment (0 μM of Paraquat) and cell induced with 40 μM of Paraquat, both presented μ= 0,18 h-1. Although the Stanford 2009 team showed that Paraquat concentrations between 60 μM and 80 μM can inhibit E. coli cell growth due to enhanced toxicity, but these concentrations were not included in our tests.

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