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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 have characterized the apoptotic potential of Bax protein, when the BAX gene is integrated into the yeast genome, as compared to when it is expressed on a high copy plasmid.

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 1000 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.

[INSERT GEL IMAGE of yeast colony PCR]

Figure 1: Yeast colony PCR of genomically integrated transformants | The positive colony of yeast is confirmed by the expected band size of around 1000 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. 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.

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

As plasmids are transposable elements, we wanted to see wether there is an advantage of using genomic integration to express the bax toxin in yeast. For this purpose, we used our constructs having bax under the inducible promoter GAL1. Cultures of yeast containing pGAL1-BAX, both expressed on a plasmid (PWUS) and genome integrated, were grown O/N and diluted to an OD_600nm of 0.5. Cultures with the respective empty vectors served as controls. On plates with different galactose concentrations (0%, 0.025%, 0.05%, 0.1% and 0.2%), 10 µl of culture were spotted in increasing dilutions (10^-1 to 10^-4) along with the control. Due to the equalised cell density of the cultures, the expression rate of the genome integrated and plasmid-expressed bax could then be compared among each other and with the controls after an incubation of three days at 30 °C.

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 OD_600nm 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.

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