Galactose dose response of Gal1 Promoter in pRS415


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Measurement of dose responsiveness of the GAL1 promoter to galactose using construct GAL1p-(Npep-GFP)


Previous dose response experiments using the fluorometer revealed that full GAL1 promoter induction was achieved at concentrations above 0.5% (data not shown). We wanted to examine the dose responsive behaviour of the GAL1 promoter across a full range of concentrations. Therefore the dose response experiments were repeated using lower concentrations of this inducing agent. We have therefore tested media containing: 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 1% and 2% of galactose.


1. Yeast transformed with a plasmid carrying the GAL1p-(Npep-GFP) construct was inoculated overnight into 5 ml of synthetic defined (SD) medium with amino acids: his (0.2 %), met (0.2 %), ura (0.2 %), trp (0.2 %) and raffinose (2 %) as the carbon source.

2. The following evening this cell culture was sub-cultured into a flask containing pre-warmed SD medium (50 mls) with 2% raffinose, and one of a range of concentrations of galactose between 0.05% and 2% of galactose, to achieve an optical density at 600nm of 0.6 by 9.00 am the following morning.

3. Samples were washed into PBS, and diluted 1/20 in preparation for FACS analysis.


Flow cytometry was used to quantify GFP fluorescence, with an excitation wavelength of 488 nm, and an emission filter of 510 nm,


The graph above summarises the FACS data, and shows that the intensity of GFP expressing cells increases in response to the percentage of galactose in the growth medium. The GAL1 promoter in our construct showed a high degree of sensitivity to the inducing agent, with concentrations as low as 0.01% having significant inducing potential.


The experiment clearly showed that the percentage of cells expressing GFP was exquisitely sensitive to the presence of galactose, with the dose response saturating above 0.1% galactose. This therefore clearly shows that the GAL1 promoter is highly sensitive, but that as a synthetic biology part, it may not exhibit ideal linear responses to inducing agent for some applications. The observed GFP expression response suggests that the GAL1 promoter behaves as an analogue switch across only a very narrow range of inducer concentrations.

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