Part:BBa_K2365048:Experience
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Applications of BBa_K2365048
Submission by Team UCopenhagen 2019:
Characterization of BAX: Genomic integration vs. plasmid expression
We further characterized the Bax protein under the expression of an inducible promoter, pGAL1 (BBa_K3190050). We chose to use this part, as it is Team NAU China 2017's improved version of the original part of Team Debrecen-Hungary 2010 (BBa_K364202).
We have characterized the apoptotic potential of Bax protein under the expression of pGAL1, when the BAX gene is integrated into the yeast genome, as compared to when it is expressed on a high copy plasmid. Furthermore, we have also characterized the neccesary concentration of inducer (galactose) needed in order for high enough expression of BAX to kill the cells.
Using USER ligation, we assembled the BAX gene with pGAL1 on a plasmid backbone compatible with multiplex integration cassette. The backbone used contains a URA selection marker, and will integrate the construct in the yeast genome at chromosome 11, site 2.
Furthermore, we also transformed yeast with a dual plasmid system. Using USER ligation, we assembled pGAL1 and BAX on a high copy plasmid backbone (200 copies/cell) containing a URA selection marker.
Yeast transformation
After cloning and purifying the pGAL1-BAX constructs in E. coli, we confirmed the sequences of these, prior to transforming into S. cerevisiae.
For the genomically integrated strain, the positive transformants were confirmed by performing yeast colony PCR. We used 2 primers, one in the forward direction for the backbone and one in the reverse direction for the yeast chromosome 11. In the presence of our construct, we expect to see a band at around 800 bp as, that is the size of the fragment between the two primer regions. In the absence of the constructs, we expect to see the bands at around 1500 bp, as this is the size of site 2 of chromosome 11.
Figure 1: Yeast colony PCR of genomically integrated transformants | The positive colony of yeast is confirmed by the expected band size of around 800 bp.
For the plasmid expressing strain, we transformed the yeast with two plasmid: the GAL1-BAX cloned in a high copy plasmid (200 copies/cell) with a URA marker, and an empty vector plasmid with a TRP marker. After transforming the yeast, we grew the colonies on plates without URA and TRP, thus selecting for positive transformants of both plasmids. To further confirm the functionality of the GAL1 promoter, we also plated the transformed yeast on a plate with 1 % galactose. As expected, we did not see any growth on the galactose positive plate, as the GAL1 promoter was induced, expressing BAX, thus killing the yeast.
Figure 2: Transformant plates of dual plasmid transformed S. cerevisiae | Both transformed with pGAL1-BAX (URA marker) and an empty vector (TRP marker). A: plate with no galactose. B: plate with 1 % galactose.
Galactose induction assay
To analyse the effect of BAX on our yeast under different expression levels, we conducted a galactose induction assay using raff-U plates with five different galactose concentrations. Cultures of yeast containing pGAL1-BAX or empty vector were grown O/N and then diluted to an OD600nm of 0.5. On plates with either 0%, 0.025%, 0.05%, 0.1% or 0.2% galactose, 10 µl of each culture were spotted in increasing dilutions (10-1 to 10-4; Figure 3 and 4). After three days of incubation at 30 °C, two observations were made. First off, the strain with the integrated pGAL1-BAX construct showed decreased growth compared to the control strain even when galactose was absent (Figure 3). This suggests that the galactose promoter is leaky and a low amount of BAX is produced at all times.
[Insert discussion of plates/results here]
Figure 3: Growth of pGAL1-BAX and the empty vector strain in the absence of galactose. The cultures were spotted in the dilutions 10-1 to 10-4 of an OD600nm of 0.5 and incubated for three days at 30 °C.
Secondly, when comparing the spots of OV6 at a dilution of 10-1 at different galactose concentrations, a clear inverse correlation of CFU/ml and percentage of galactose in the media can be observed (Figure 4). This suggests that successful induction of BAX leads to apoptosis in our yeast.
Figure 4: Growth comparison of OV6 and control strain OV1 in the presence of varying galactose concentrations. | Shown are the CFUs at an OD of 0.05 after incubation at 30 °C for four days. The three yellow colonies seen at 0.2% galactose in OV6 can be morphologically distinguished from the others, suggesting that they are contaminants.
Quantitative galactose induction assay
To quantify the effect of bax on our yeast under different expression levels, we have conducted a quantitative galactose induction assay with the genome integrated pGAL1-BAX using varying concentrations galactose. For this, an O/N culture of yeast with pGAL1-BAX was diluted to an OD600nm of 0.5. Subsequently, 100 µl of the culture were spread on plates with 0%, 0.05%, 0.1%, 0.3% and 1% galactose, respectively. To obtain countable amounts of colonies, the culture was spread in 10-3 and 10-4 dilutions. Yeast containing the corresponding empty vector served as a control. After incubation for three days at 30 °C, the CFU/ml were calculated and compared to the control.
[Discussion of below graph]
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