Part:BBa_K201001:Experience

To experimentally and mathematically characterize the device and its sensitive to the inducer, we studied both the static and a dynamic response to IPTG induction.

Dh5alpha cells were co-transformed with the BBa_K201001 on a high copy number plasmid (pSB1A2) and BBa_K201002 on a low copy number plasmid (pSB3K3).

Static response Dh5alpha were inoculated in 5 ml of M9 medium with 0, 10, 20, 40, 60, 80, 100 uM IPTG, respectively. After O/N growth at 37° (about 12 h) samples were collected and slides prepared for microscope analysis. Images were then analyzed with the [http://2009.igem.org/Team:Bologna/Software VIFluoR software]. To obtain a significant representation of bacterium fluorescence, it was necessary to acquire several images, each one reporting a sufficient number of bacterial cells (n=60). VIFluoR operates the image segmentation and then recognises the bacterial cells yielding the mean fluorescence per bacterium as the output. The experimental data (Fig. 1) were used to identify, by the [http://2009.igem.org/Team:Bologna/Modeling mathematical model], the operator binding affinity for the repressor LacI (K= 1.7 nM).

Fig.1. Experimental data (blue lines) of the static induction after 0, 10, 20, 40, 60, 80, 100 uM IPTG induction. Data were fitted by the model (green line) to identify the operator-repressor binding affinity (K= 1.7 nM)

After parameter identification, we computed by the model the static control curve for the LacI repressed GFP generator (LacI inverter) (Fig.2).

Fig.2. Model prediction of promoter repression by Lac I.

Dynamic response Dh5alpha cells were inoculated in the morning (9 a.m.) in 5 ml of M9 medium with no IPTG. After daily growth (about 8 h) the culture was diluited to an OD=0.1. To perform the induction analysis, the culture was splitted in two. A half was induced with 100 uM IPTG and the other was grown in control medium. 200 ul of each sample were used to fill plate wells (6 wells each). Cells were grown into a fluorimeter (Tecan M200) O/N (about 12h) at 37°. OD and fluorescence were sampled each 15 min (Fig. 3 and 4, respectively).

Fig.3. Growth curve for the uninduced (black line) and induced (100 uM IPTG, red line)system.

Fig.4. Absolute fluorescence curve for the uninduced (black line) and induced (red line, 100 uM IPTG)system.

Fig.5. Model fitting of the experimental data. Experimental data (blue lines) were fitted by the model considering a constant (red line) or a varying (green line) amount of RNA polymerase

Fluorescence over OD was compared with the model prediction (Fig. 5) considering both a constant and a varying amount of the RNA polymerase enzyme. A good fitting can only be obtained if a varying Polymerase is consider. This can be explained considering the GFP expression depending on the growth phase.

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