Part:BBa_K733018
xylR+PxylA+RBS+GFP+Double Terminator
This construct is built to measure the relative efficiency of xylose inducible promoter in E. coli. We intend to use GFP as an indicator to characterize this xylose inducible promoter derived from Bacillus megaterium (BBa_K733002). In our characterization, we will investigate the influence of concentration of xylose on xylose inducible promoter and use GFP intensity changes over certain period of time to represent the regulatory effect of this promoter in different xylose inducing conditions.
Characterization
Background Information [http://2012.igem.org/Team:HKUST-Hong_Kong/Module/Regulation_and_control (link to our Regulation and Control Module)]
The reason for using the xylose inducible promoter is to enable control on the expression of toxin and BMP2. Xylose is not toxic and normally not present in the human colon. This provides us an easy way to induce BMP2 expression without disrupting normal human body function.
Objective
On characterization, we want to test whether the promoter works in E. coli DH10B strain and if it works, what is the absolute promoter activity under varied experimental condition (i.e. xylose concentration).
Intended Result
1. Xylose inducible promoter is functional in E.coli.
2. After the inducer concentration has reached certain level, a relatively stationary GFP expression level (expression upper-limit should be observed.
Method
The absolute promoter activity was measured with respect to xylose concentration.
The same reporter gene (BBa_E0240) was used to indicate promoter activity. E. coli carrying the right construct was cultured to log phase. Following the addition of xylose at various predetermined concentrations, at a time point around the mid-log phase, the GFP intensity and OD595 were measured for every 30 mins (up to 120 mins). Independent curves indicating the GFP intensity units (of various xylose concentrations) with respect to time were then plotted, following which the respective absolute promoter activities were calculated.
Characterization Procedure
1. Constructing xylR-PxylA-BBa_E0240-pSB1A2;
2. Preparing supplemented M9 medium (see below);
3. Culturing E. coli carrying xylR-PxylA-BBa_E0240-pSB1A2 and E. coli without constructs in supplemented M9 medium and measuring the growth curve respectively;
4. Culturing the above mentioned bacteria in supplemented M9 medium to log phase;
5. Adding xylose at different concentrations to different sets of bacterial culture;
6. Measuring the GFP intensity and OD595 values across time for every set of bacterial culture containing different xylose concentrations;
7. Plotting independent curves showing the GFP intensity units of various xylose concentrations with respect to time;
8. Plotting a graph to demonstrate the absolute promoter activity under different inducer concentrations;
9. Compiling the results.
Data Processing
1. After E. coli carrying the right construct was grown to mid-log phase, GFP intensity and OD595 were measured every 30 minutes (up to 120 mins);
2. For GFP intensity, curve reflecting GFP expression change was plotted; for OD595, average value was taken;
3. GFP synthesis rate was then obtained by calculating the slope of linear regression line of the above mentioned curve;
4. Absolute promoter activity for the promoter under different inducer concentrations were calculated by dividing the corresponding GFP synthesis rate over the average OD595 value;
5. Averaged absolute promoter activity was then obtained by averaging the respective sets of absolute promoter activity values.
Result
1. Shown in the figure below, with the addition of xylose, GFP expression increased. This tells us that the xylose inducible promoter is functional in E. coli DH10B strain.
2. When no xylose was added, a limited amount of GFP was expressed. This suggests that the xylose inducible promoter is to some extent leaky.
3. A relatively stationary GFP expression level was observed at xylose concentrations of 1% to 5%. Despite some other variables (see discussion for more details), the data suggests that the minimum inducer concentration for triggering a full induction should lie somewhere between 0% and 1%.
4. For 10% inducer concentration, the GFP expression was relatively lower. There could be several reasons for this occurence, such as suboptimal growth conditions due to high osmotic pressure. (See discussion for more details.)
Discussion
1. It is quite obvious that addition of xylose induces GFP expression in this construct. However, a slight issue remains: even when no xylose was added, a minute but detectable amount of GFP was still expressed. This shows that the xylose inducible promoter is leaky. It should be noteworthy that this version of the xylose inducible promoter has undergone mutagenesis on 3 different sites on the repressive gene for standardization purposes. Even so, the mutagenesis is done while preserving an identical codon translation. As such, similar to the Ptms promoter, activity of the xylR-PxylA promoter might be different in E. coli from that of B. subtilis due to expression in a heterologous system. Further characterization on this promoter in B. subtilis would be our future aim as time was limited.
2. Another interesting fact to note is that the E. coli strain used is one in which it's xylose metabolic operon remains active. As such, one might assume that the observed GFP expression upper-limit ("stationary expression level"),particularly at 1% or 2% is not the "true" upper-limit, since there should be underlying metabolism of xylose. In order to eliminate this possible error, higher concentrations of xylose was used and the promoter activity does not appear to vary greatly, suggesting that the stationary expression level reflects the maximum promoter activity. While this problem is solved, due to the relatively large concentration difference of inducer, it appears that the maximum promoter activity was achieved at 1% xylose, implying that we are unable to determine the exact minimum inducer concentration requirement for maximum activity. Based on the current result, it is safe for us to make a conjecture that the minimum concentration required lies between 0% and 1% xylose.
3. For the decreased GFP expression at 10% xylose concentration, one possible reason is that the high osmotic pressure caused by the xylose in the medium may inhibit the growth and metabolism of bacteria, thus reducing the bacterial population and/or its GFP expression. Another possible, but unlikely reason could be that the over-expression of induced GFP expression may disturb the normal bacterial function, leading to a low overall GFP expression.
Supplemented M9 Medium Composition
1. 5X M9 Salt Composition (1L)
(1) 64g Na2HPO4﹒
(2) 15g KH2PO4
(3) 2.5g NaCl
(4) 5.0g NH4Cl
2. Minimal 1X M9 medium (1L)
(1) 200ml of 5X M9 Salt
(2) 2ml of 1M MgSO4
(3) 100μl of 1M CaCl2
(4) 5ml of 40% glycerol
3. Supplement (for the final medium)
(1) 1mM thiamine hydrochloride
(2) 0.2% casamino acids
Sequence and Features
- 10COMPATIBLE WITH RFC[10]
- 12COMPATIBLE WITH RFC[12]
- 21INCOMPATIBLE WITH RFC[21]Illegal XhoI site found at 847
- 23COMPATIBLE WITH RFC[23]
- 25COMPATIBLE WITH RFC[25]
- 1000INCOMPATIBLE WITH RFC[1000]Illegal BsaI.rc site found at 2058
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