Part:BBa_K554002:Experience

HlyA secretion signal peptide

This part was used to [http://2011.igem.org/Team:UNICAMP-EMSE_Brazil/Results#Device_3_testing.2C_Protein_Secretion_System Device 3 testing, Protein Secretion System]

Device 3 testing, Protein Secretion System

The assembled devices related to secretion system (Hemolysin secretion system under control of SoxS – SoxS-HlyB-HlyD-TolC) was tested under laboratory conditions using GFP as reporter.

Figure 1: Testing [http://2011.igem.org/Team:UNICAMP-EMSE_Brazil/Project#Device_3:_Secretion_system Device 3] through replacement of IL-10 to GFP. This is a representation of device 3, the protein secretion system coupled to Sox regulated GPF production device (Part BBa_K554012). To export a protein, the bacteria must have the HlyD, HlyB and TolC proteins and the target protein must have a signal sequence (HlyA tail), as shown here in a light red oval. In this case, the target protein to be secreted is GFP.

Methods

To access GFP secretion, competent E. coli DH5α strain cells were transformed simultaneously with a pSB1C3 vector (Chloramphenicol resistant) carrying both the sensor (Strong_Constitutive_promoter + RBS + SoxR + Terminator) and the effector (SoxS_promoter + RBS + GFP + HlyA + Terminator), and pSB1AK3 (Ampicillin resistant) carrying the secretion system (SoxS_promoter + RBS + HlyB + HlyD + TolC + Terminator). As a non-secretion control, E. coli harboring only the pSB1C3 vector with both sensor and effector systems was used.

Oxidative stress was induced by adding increasing concentrations of Paraquat (Methyl viologen dichloride hydrate - Sigma), an oxidative stress inducer in bacteria. The secretion was tested in cultures harboring A) and B) (Figure ) using 0 μM and 40 μM of Paraquat as described [http://2011.igem.org/Team:UNICAMP-EMSE_Brazil/Results#Methods above]. After 3 hours samples were collected, centrifuged (4000 rpm / 10 min; to avoid cell lysis) and the supernatant was collected and centrifuged again (13000 rpm / 10 min; to remove remaining cells). The supernatant fluorescence was measured in fluorometer (SLM – Aminco; 4 nm bandpass and 10 mm) with excitation in 500 nm and emission spectra from 508-550 nm. The GFP fluorescence was also detected in cells by fluorescence microscopy (Olympus).

Results

GFP secretion was confirmed by fluorescence emission and estimated to be approximately 6% of total protein, according to fluorescence levels. Significant levels of GFP fluorescence were found only in the supernatant of Paraquat induced cultures containing both the sensor/effector and the secretion systems but not in the non-induced cultures and in the cultures containing only the sensor/effector system (Figure 2).

In the black line we have our control, bacteria with the complete secretion system and without paraquat added. You see low fluorescence levels. The red line represents the culture induced with 40uM paraquat, with the fluorescence peak at 511nm. In order to check it this fluor. Peak is due to na artifact such as cell lysis we tested the cultures without secr. System with paraquat added. We saw no fluor. Peak and we may conclude that the observed for red is really due to GFP secretion.

Figure 2: Supernatant fluorescence emission from 508 to 540 nm, excitation in 500 nm. Red line (40 μM Paraquat added): sensor/effector with secretion system, showing a clear peak of fluorescence measurement at 511 nm; Black line (0 μM): sensor/effector with secretion system, non-induced. Blue line (40 μM Paraquat added): sensor/effector without secretion system;

From Figure 2, it is possible to observe that the red line shows a clear and high supernatant fluorescence peak at 511 nm, and the values are higher than the bacteria harboring the three systems (NO sensor, GFP producer and protein secretion) but whithout induction of NO by Paraquat (Black line), which may indicate that GFP is being exported. To test if the observed effect is not a False Positive (e.g. the same effect could be observed when bacteria suffer a cell lysis or if there is a secretion system in bacteria that could promote GFP export to the extracelular media) we decidede to check the supernatant fluorescence of bacteria with 40 uM Paraquat added and harboring only two systems: the NO sensor and GFP production devices but lacking the secretion system device (Blue line). This experiment showed that the fluorescence levels found in the Blue line are significantly lower than the observed for the red lines, which may indicate that the observed pattern of red line is not due to an error.

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