Part:BBa_K911009:Experience

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Applications of BBa_K911009

Our initial construct used pSPAK as our inducible (i.e. parameter sensitive) promotor. The construct we made can be seen below:

Graph of OD600/YFP and time during exponential phase. Note that the OD600/YFP ratio reaches a constant value near the end of exponential phase, indicating that it is a good indicator of cell activity - this ratio gives a measure of the YFP per cell, which is constant.

Initial assays were performed with just the construct and no induction of the promotor in order to check the reliability of our ratiometric output. These initial assays gave promising results; YFP increased dramatically during exponential phase, as did CFP to a lesser extent. Given that CFP was not being driven by IPTG, it was not surprising that this should be the case. The OD600/YFP ratio also settled down to a constant value for all cultures at the end of exponential phase, indicating that our YFP channel was a reliable indicator of cell activity in constant conditions.

However, as we moved on to checking our construct worked for measuring the concentration of IPTG, we noticed some problems. While our construct did produce a constant ratio of CFP/YFP in our cultures during steady state (representative samples of this ratio taking can be seen), the ratio during exponential phase changed rapidly. We have reason to believe that this may have been due to the short amount of time between induction and the end of exponential phase (~4hours, as opposed to a more ideal 15 or so). In this short time span, the fluorescent proteins already present in the cells may not have had time to 'dilute' out upon successive cell divisions.

Graphs of YFP normalised fluorescence vs. CFP normalized fluorescence for three different IPTG concentrations. Note the extreme linearity, indicating that our internal ratiometric control is working.

Nevertheless, we hypothesized that the ratio during steady state may be a good indicator of IPTG concentration. To determine this ratio, we assumed that accumulation of fluorescent proteins during steady state was constant, as production was constant and degradation in this time period was negligible. Making these assumptions, simply taking the gradient of the linear regression of the CFP intensity vs. YFP intensity gave a readout of the

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