Part:BBa_K1583101
pRha + CsgA + His-tag
CsgA with HIS-tag attachted to the C-terminus under control of L-rhamnose-inducible promoter ((BBa_K914003). CsgA is a protein monomer which can aggregate to form amyloid nanowires in natural biofilms of E.coli. This protein is transported as an unfolded protein out of the cell. Outside the cell CsgA proteins self-assemble into nanowires after nucleation on the membrane protein CsgB. CsgC prevents CsgA proteins from self-assembling inside the cell and the transport is ensured by the proteins CsgEFG.
Characterization
Four different experiments were done to characterise the BBa_K1583101 biobrick:
Fluorescence assay
To be able to ensure that CsgA is expressed, we added a gene encoding for GFPmut3 (BBa_I13504) under induction of the same rhamnose promoter (BBa_K1583112) to check that the promoter works. In this experiment, the fluorescence signal of our csgA construct and csgA-GFP (I13504) constructs was recorded in time after induction with no, 0.2% (w/v) or 0.5% (w/v) rhamnose. Besides the fluorescence, the OD600 was measured in order to normalize the fluorescence signal per cell.All conditions were carried out in triplicates to be able to do a statistical analysis on the data. The different experiments were induced in a 96 well plate. The OD600 and fluorescence signal was recorded in a plate reader during a 18 hour period of induction at 30°C.
In Fig. 1, the fluorescent signal was normalized by the number of cells and plotted as a function of time. The red bars denote the error within each ID.
As can be seen from Fig. 1, only the experiments with 0.2% (w/v) and 0.5% (w/v) rhamnose induction with the csgA-GFPmut3 construct gave a clear increase in fluorescence signal in time. All other experiments, gave similar levels of fluorescence, slightly increasing in time. Furthermore, it can be seen that a higher induction level of rhamnose leads to an increase in GFPmut3 and thus fluorescence. Finally, as the fluorescence signal is normalized by the cell density, one can make statements about the activity of the rhamnose promoter. The promoter seems to not be active right after induction, but more after 3 or 4 hours. This is in accordance with data from literature (Wegerer et. al), in which a low amount of fluorescence with a rhamnose promoter was observed after 2 hours of induction.
With this kinetic experiment, we have proven that the rhamnose promoter does indeed induce the expression of the csgA gene.
Crystal violet assay
The assay above showed that the bacteria that we engineered for the project is capable of producing the CsgA proteins after induction with L-rhamnose. However, this did not yet prove that curli are formed. In order to assess whether our bacteria producting CsgA with a His tag can still produce these nanowires, our team adapted the protocol from Zhou et al. (2013) that employs crystal violet (methyl violet 10B) for dying the biofilm-making bacteria that attaches to the surface. In the experiment, our CsgA_His-producing strain of E. coli was induced at a high (0.5% w/v), low (0.2% w/v) and no (0% w/v) concentration of L-rhamnose. Furthermore, csgA deficient bacteria transformed with an empty plasmid (pSB1C3) were used as control. In the end, the wells were diluted with ethanol so all the content can dissolve in the liquid phase. We measured the absorbance at 590 nm of wavelength for all the samples, obtaining the following results (figure 3.).
The CsgA_His proteins seem to have retained the capability to form curli despite having a peptidic modification on the C-terminus. To confirm that there is a real change between the analysed samples and the empty plasmid control, a significance analysis was performed for α=0.05 (Table 1.).
| Sample | p-value | Significant difference (5%) |
|---|---|---|
| CTRL2 & HIST2 | 0.0242 | Yes |
| CTRL5 & HIST5 | 0.0026 | Yes |
The significance analysis shows that cells containing the CsgA_His (BBa_K1583101) biobrick can efficiently create a curli, when compared with an empty plasmid control (i.e. without csgA expression).
Western blot
For our modeling, we needed to determine the internal concentration of csgA. Based on the fluorescent assay with GFPmut3, we obtained an internal amount of CsgA in the order of 104 molecules/cell. To validate whether this value was plausible we decided to use Western Bloting. The following strains were used (Table 2.) (ΔcsgA is a csgA deficient strain called E. coli K-12 MG1655 PRO ΔcsgA ompR234). https://static.igem.org/mediawiki/2015/f/fc/TU_Delft_2015_tab3.png
The procedure of this experiment is described here at Protocols > "Isolation using Ni-column purification assay (QIAGEN kit) for extracellular protein tagged with 6xHis3".
The SDS gel is shown in figure 4. The gel is too condensed to visualize a difference between the non-induced and induced samples, because the gel was overloaded.
Transmission electron microscopy
Using TEM the formation of curli of the biobrick BBa_K1583100 was visualized. Although this is a different biobrick (no addition His-tag), curli formation can be presumed to be similar for this biobrick.
We did not observe formation of curli nanowires in the uninduced cultures of our strain. However, cells from induced cells clearly produced them, as supported by the TEM images.
Sequence and Features
- 10COMPATIBLE WITH RFC[10]
- 12COMPATIBLE WITH RFC[12]
- 21INCOMPATIBLE WITH RFC[21]Unknown
- 23COMPATIBLE WITH RFC[23]
- 25COMPATIBLE WITH RFC[25]
- 1000COMPATIBLE WITH RFC[1000]
Reference
Optimization of an E. coli L-rhamnose-inducible expression vector: test of various genetic module combinations”, Angelika Wegerer, Tianqi Sun and Josef Altenbuchner, BMC Biotechnology 2008, 8:2
Zhou, Kang, Kangjian Qiao, Steven Edgar, and Gregory Stephanopoulos. 2015. “Distributing a Metabolic Pathway among a Microbial Consortium Enhances Production of Natural Products.” Nature Biotechnology 33(4): 377–83.
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