Revision as of 11:04, 24 September 2012

PyeaR + CArG nitric oxide, nitrate & nitrite-sensing hybrid 'Mammalian-Bacterial' promoter

NRP-UEA-Norwich 2012

The team (NRPUEA 2012) decided to develop the PyeaR biobrick (BBa_K216005) further by ligating it to its mammalian counterpart: CArG promoter sequence E9-ns2 (Scott, S.D. et al.2002). To provide additional restriction enzyme sites that may become useful during later cloning steps, BamHI, HindIII and NdeI were added between the 2 promoters. The genes were synthesised in two orientations, bacterial-mammalian (BBa_K774000) and mammalian-bacterial (BBa_K774001) as initially we were not sure what effect gene order would have on gene activity. The aim of this development was to increase the flexibility of the PyeaR promoter so that it can be used in both mammalian and bacterial systems. This is something that we thought was important as sensing nitric oxide in the human body has a wide range of therapeutic applications (please see the future applications section on our wiki).

The characterisation of our biobricks was carried out as follows:

-Growth studies of the PyeaR biobrick (BBa_K381001), the mammalian-bacterial (M-B) biobrick (BBa_K774001), and the bacterial-mammalian (B-M) biobrick (BBa_K774000).

-Measuring the fluorescence of the M-B biobrick ligated with Red Flourescent Protein (RFP) and enhanced Cyan Fluorescent Protein (eCFP), as well as the B-M biobrick ligated with RFP and CFP, in response to induction with different potassium nitrate concentrations.

-Measuring the number of cells which fluoresce in different potassium nitrate concentrations using flow cytometry.

Growth Studies

A comparison between the growth of E.coli cells, before and after transformation with the bacterial-mammalian promoter, as well as the mammalian-bacterial promoter (BBa_K774001) and PyeaR + GFP composite (BBa_K381001))

The study involved testing the affects of transforming E.coli with different promoters on its growth over time. The promoters E.coli had been transformed with were PyeaR, M-B and B-M. These are promoters which all react to nitrogenous species. By running these together, we can obtain a direct comparison between all three of these promoters on the growth of E.coli. To see if there are any significant changes, the study was run alongside E.coli cells which had not been transformed with anything. For the rest of this brief report, untransformed cells will be referred to as Alpha cells and the other E.coli cells with transformations will be referred to as the promoter with which they were transformed with.

The E.coli cells used in the study and for the transformation are the same type of cells (Alpha select gold standard cells from Bioline). A colony was inoculated into 5ml of LB media overnight and the cells spun down the following morning and diluted with fresh LB until an OD reading at 600nm of 0.2 ± 0.01 was obtained. Three repeats were made of each sample.

The study lasted for 12 hours. An OD reading at 600nm was taken once an hour. Between the hour, the cuvettes were put into a 37ᵒC incubator to encourage growth and for standardising measurements with other growth studies. To calculate the number of cells in the samples, a calibration curve was set up. This involved using cultures of the E.coli cells without transformations. The E.coli cells were diluted with different volumes of LB and OD readings were taken as well as plating on Agar plates. After a day of growth, the numbers on these plates were counted and recorded. The CFU/ml was calculated. When the OD readings (x axis) and the CFU/ml (y axis) readings are plotted, the equation of the line of best fit, gives a conversion for the absorbance readings. This allowed us to measure the growth. This is demonstrated in figure 1.

Figure 1. Calibration curve to calculate the conversion factor between OD reading at 600nm and the number of colony forming units growing per ml (CFU/ml)

We found that there was a significant difference between Alpha cells and PyeaR cells. Initially, Alpha cells had a greater growth rate, but after the third hour into the study, the growth rate of PyeaR was faster than that of Alpha cells. The overall growth rate of PyeaR cells was significantly faster that Alpha cells (Levenes Test, F = 1.009 p = 0.372; T Test, t = 4.196, df = 4, p = 0.014).

Figure 2. Growth of PyeaR transformed E.coli cells relative to Alpha cells (untransformed cells). Error bars show the standard deviation between the three repeats. For clarity reasons, lines of best fit are not shown The growth pattern and rate of E.coli cells with or without transformation with B-M and M-B show little difference. Any differences in growth rate were not significant. There was lots of overlap. As previously described, there was a significant difference between the growth rate of PyeaR and Alpha cells. There was also a significant difference between MB/BM and PyeaR cells. The statistical results can be seen in Table 1.





Figure 3.Growth over 12 hours of Alpha, M-B and B-M. Error bars and lines of best fit are not shown for clarity reasons.

Table 1. ANOVA readings of statistical differences between Alpha (1) PyeaR (2), MB (3) and BM (4).

From all the above graphs, it can be seen that with the starting concentration of cells as high as they are, the cultures are in exponential stage and do not undergo lag phase. A further growth study will be carried out on purely the lag phase with lower starting concentrations. As the starting absorbances here are approximately 0.2 at a wavelength of 600nm, the lag phase study will involve starting absorbances of 0.04 and lower.

A comparison between the growth of E.coli cells, before and after transformation with PyeaR + GFP (BBa_K381001) and B-M and M-B (in pSB1C3)- Lag Phase Study

Following the above study, we found that a lag phase only study needed to be carried out to see if there was a significant difference in the lag phase. Again the study protocol was the same except that the starting concentration absorbances at 600nm was lowered to <0.04. It was extremely difficult to keep the absorbances ranges within 0.005 so the range is actually 0.3±0.1. The below graph shows the mean average of all the data; using the data from the calibration curve, the absorbances were converted to colony forming units per ml (CFU/ml). The trend lines of alpha cells, BM/MB and PyeaR transformed cells are shown within this order from highest to lowest trendlines. One single trendline was used to represent BM and MB because the actual trendlines were extremely similar. Using the initial concentrations of 0.3±0.1 showed that there is little difference between the growth rates. Using statistical analysis, it was found that there was no significant difference between any of the transformed cells relative to Alpha cells or to each other (Anova, p > 0.05).

From this study we have found that changes in growth occur during exponential growth phase and not the lag growth phase.

Ligating the mammalian-bacterial promoter to fluorescent proteins

The hybrid promoter was been ligated to two different reporters: enhanced Cyan Fluorescence Protein (eCFP)(BBa_K774006) and Red Fluorescent Protein (RFP) (BBa_K774007). The hybrid promoter was characterised by observing expression of flourescent protein, and found to have increased transcription in response to increasing concentrations of potassium nitrate.



The graph above shows the flourescence measured from the expression of eCFP due to the response of the mammalian-bacterial promoter to different concentrations of potassium nitrate. The wavelength reading which corresponds to eCFP is between 440-500nm. The graph clearly demonstrates that between 0mN and 15mM there is a proportional relationship between fluorescence intensity and potassium nitrate concentration. It can be noted that at a 20mM concentration the intensity of fluorescence sharply decreases back down to the level of 5mM potassium nitate concentration. This may be due to the cell overexpressing eCFP up to the point at which the excess protein begins to form inclusion bodies which can no longer fluoresce; alternatively, this could be due the potassium nitrate concentration reaching the critical concentration at which it becomes toxic to the cell. This data differs to the readings taken from the bacterial-mammalian promoter ligated to eCFP, as well as the hybrid promoters to RFP, which may suggest there is a difference in the molecular mechanisms that these promoters function by; however at this point the change in intensity at 20mM is inconclusive and is an area which we would like to look into further.

The graph above shows the flourescence measured from the expression of RFP due to the response of the mammalian-bacterial promoter to different concentrations of potassium nitrate. The wavelength reading which corresponds to RFP is between 600-650nm. The graph clearly demonstrates that between 0mN and 15mM there is a proportional relationship between fluorescence intensity and potassium nitrate concentration. It has been found that for all biobricks apart from the mammalian-bacterial promoter ligated to eCFP at a 20mM concentration the intensity of fluorescence sharply decreases. This may be due to the cell overexpressing eCFP up to the point at which the excess protein begins to form inclusion bodies which can no longer fluoresce; alternatively, this could be due the potassium nitrate concentration reaching the critical concentration at which it becomes toxic to the cell. This data differs to the readings taken from the bacterial-mammalian ligated to eCFP, as well as the hybrid promoters to RFP, which may suggest there is a difference in the molecular mechanisms that these promoters function by; however at this point the change in intensity at 20mM is inconclusive and is an area which we would like to look into further.

Flow cytometry was used with part BBa_K774006 to quantify the number of cells which fluoresced in response to induction by potassium nitrate. As the graph (right) indicates, the number of cells which fluoresce is proportional to the concentration of potassium nitrate that the cells are exposed to.

Refernces

Scott, S.D., Joiner, M.C. & Marples, B., 2002. Optimizing radiation-responsive gene promoters for radiogenetic cancer therapy. Gene therapy, 9(20), p.1396-402. Available at: http://www.ncbi.nlm.nih.gov/pubmed/12365005 [Accessed June 24, 2012].