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Stability

Induction time to achieve 42ºC state stability

First, we wanted to determine the 42ºC induction time necessary to make the state change from "IPTG state" to "42ºC state" stable (see the note at the end of the page). For this purpose we induced the state change of a culture previously established in the IPTG state by heat shock at 42ºC during different times: 2h, 3h, 4h and the whole assay (7h aprox.). After heat shocking, bacteria were grown at 30ºC. Red and green fluorescence levels were measured during 7 hours, including the heat shock time. The results are shown in figure 5. This assay was performed using a MC4100 E. coli strain.

Figure 5. Relative fluorescent level of the improved flip flop with different temperature inductions time lapses. These heat shocks start at minute zero and finish at the point indicated by the arrows. In figure 1D, this heat shock is maintained during the whole experiment. It can be seen that 2 and 3 hours heatshock is not enough to maintain the 42ºC state stability, as predicted by the mathematic models. However, we can see that a 4-hour induction shows a higher stability of this state.

42ºC state stability

In order to compare the 42ºC state stability of the basic and improved flip-flop, this state was induced by harvesting bacteria cultures at 42ºC during 11 hours. This long incubation period was used to effectively achieve the cultures in the 42ºC state at the beginning of the measurements. At the “Induction Stop” point, of the graphics below (Figure 6), the cultures of basic and improved flip-flop were set at 30ºC (without IPTG) and fluorometry data was taking during six and a half hours. In these graphics it is also shown how the bacteria reach the 42ºC state after change their state. This assay was performed using our previously constructed X90 (SspB, RybB double deletion) E. coli strain expressing the improved flip-flop (module I and II) and the basic flip-flop with an empty plasmid (as control).

Figure 6. Fluorescence/O.D. of basic and improved flip-flop after 42ºC induction during 11 hours (before “Induction Stop”) and without any induction during 6,5 hours (after “Induction Stop”). We had to adjust the fluorescence level of the basic flip-flop, dividing the green fluorescence intensity by 7, as in this system the green fluorescence intensity is 7-fold higher than the red fluorescence intensity. This readjustment was not necessary for the improved flip-flop due to the fact that the basal intensity levels of the fluorescent proteins are similar. In the graphics we can see that the 42ºC state stability is markedly higher in the Improved Flip-Flop compared to the Basic Flip-Flop, and that the difference between the fluorescence levels of both states is also much more pronounced.

We have showed by fluorometry measurements that our improved flip-flop increases the difference between the fluorescence levels of opposite bistable states, lowering the repressed state levels to almost zero . We have also demonstrated that the stability of the improved flip-flop is much higher than the basic flip-flop under this stimulus.

IPTG state stability

We have collected a huge amount of data to determine IPTG State Stability using flourometry and flux cytometry. Unfortunately we haven’t been able to analyse all of them yet. Though, we have observed that the “IPTG state” tends to impose itself over the 42ºC state in the absence of induction. We expect to show these results in the World Championship Jamboree.

Note: For an easier understanding of the bistable system we denominate "IPTG state" to the transcriptional state caused by IPTG induction; and "42ºC state" to the transcriptional state caused by 42ºC heat shock induction. In the improved flip-flop the 42ºC state causes GFP expression and the IPTG state causes RFP expression. The Basic Flip-Flop works in the opposite way: the 42ºC state causes RFP expression and the IPTG state causes GFP expression. The 42ºC/IPTG denomination will be very useful for the comparison of basic and improved flip-flops.