Team:ETH Zurich/Experiments/Heat Sensor

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Heat Sensor Experiments

Introduction

We incorporated a module into our system that would allow our engineered bacteria to sense the external toxin release signal of the doctor. The signal is produced with focused ultrasound and increases the temperature in the tumor region to 45 °C. To detect this signal, we needed to fit a naturally occurring heat sensor from Salmonella into our novel genetic circuit. If the heat sensor detects the temperature increase, it should activate the next step (cell lysis), by promoting transcription of protein E.

Figure 1. The genetic mechanism of our TlpA heat sensor. TlpA dimers bind in the P_TlpA region and repress transcription of the downstream gene. A temperature of 45 °C shifts the equilibrium of dimerization towards monomers, which don't bind in the P_TlpA region. Transcription can therefore happen at 45 °C.

For more details about the mechanism, go to Heat Sensor.

Phase I: Initial System Design

The TlpA heat sensor consists of two parts: the TlpA coding sequence and the P_TlpA promotor sequence. We copied the digital sequences from a plasmid used by Piraner et al. and introduced 8 silent mutations to remove forbidden restriction sites inside the coding sequence. We designed test-plasmids to ensure proper function of the system. The initial design of a test system constists of two plasmids, one containing the repressor and the other the promotor and a reporter protein. The first plasmid was designed to have an Anderson Promotor with relative strength of 0.71 that promotes transcription of an RBS (designed with Salis Lab RBS calculator) with a calculated translation initiation rate of 5000 and the TlpA coding sequence.

Figure 2. The heat sensor test plamids. The TlpA coding sequence is placed on the piG17-2-002 (pSEVA361) and the heat inducible gfp is placed on piG17-1-005 (pSEVA291). PF and SF are abbreviations for BioBrick Prefix and BioBrick Suffix restriction sites. RS1-RS4 are restriction sites that we introduced for later cloning.

The optimal amount of the TlpA repressor protein in the cytoplasm was not known to us from the beginning, that's why we decided for a medium amount of TlpA expression. The physical DNA sequences were ordered as gBlocks from IDT and inserted to our test plasmids pSEVA361 and pSEVA291 via Ligase Cycling Reactions.

Key questions to answer in first experiments

• Does the heat sensor work in our Lab with our experimental setup?
• Does the heat sensor work despite the changes we made to the coding sequence, the calculated ribosome binding site and the BioBrick promotor?
• Which temperature is needed to activate the heat sensor?
• How long does the heat sensor have to be induced for decent expression of the regulated gene?

Phase II - A: Function test with reporter gene

A sequence of experiments was performed to find optimal induction times and experimental setup. E. Coli Top10 chemical competent cells or Nissle electrocompetent cells were used. Single colonies of double transformants were inoculated to 12 mL round bottom culture tubes in 5 mL LB and grown for 16 h at 37 °C shaking 230 rpm. After growth to stationary phase, they were diluted to OD 0.1 and grown to exponential phase (OD 0.4). At this point the induction procedure was initiated in different formats, for different times and temperatures.

Our findings were:

• Induction times of 1 to 15 minutes don't induce the reporter gene strong enough, even though temperatures above 42 °C lead to slightly higher fluorescence after 15 min induction
• Strong induction takes place in a timescale of 1-5 hours. (more would not be feasible for our application)
• Growth in 12 mL culture tubes is best suited and the samples should be diluted before plate reader measurement.

Figure 3. Fluorescence readouts of the TlpA Heat sensor after induction times from 0.5 to 5 h. Biological triplicates were measured and the standard deviation is depicted. Longer induction times lead to higher fluorescence values. Even though induction is significant, the fold change is very low. Induction was performed in 60 uL cell culture in 100 uL PCR tubes in thermocyclers for the time and temperature indicated and after induction stored at 30 °C until start of the readout (5 h after start of induction).

The experimental setup was changed from induction in thermocycler to induction in shaking incubators. Shake flasks were used and the same experiment repeated.

Figure 4. Fluorescence readouts of the TlpA Heat sensor after 3h induction at 45 °C in shake flasks in a shaking incubator. Biological triplicates were measured and the standard deviation is depicted. The fluorescence of the supernatant was also measured to find out if we can use supernatant fluorescence in future cell lysis measurements.

These experiments showed that induction is possible, but the fold change of ~6 is very low. This is either due to a low maximum induction of the promotor compared to a constitutive promotor of gfp, or to a high expression when not induced. We hypothesized that the RBS of TlpA was not inducing translation enough, and low amounts of the TlpA reporessor protein don't inactivate base level expression enough. Double transformations with protein E under the TlpA operator suggested that we need a tighter regulation (they did not yield any transformant colonies due to insufficient inhibition of protein E expression leading to cell lysis).

Reducing the heat sensor's leakiness

It is very important for proper function of CATE to have a very tightly controlled activation of cell lysis. Only if CATE releases the anti-cancer compound upon the external heat signal an improvement of the precision of toxin delivery is possible. Additionally, it is not possible to transform E. coli with a heat inducible toxic compound with such a leaky expression - the leaky protein E expression already kills all transformants.

TlpA RBS library creation

A ribosome binding site library was then created to find variants translating more TlpA RNA. The Red Libs algorithm was used and set to calculate degenerate sequences that produce 12 variants. The variants should all have a rather high expression rate to increase the cytoplasmic amount of TlpA dimers able to repress the promotor. Degenerate primers were ordered at Microsynth and the library was created with a simple PCR and subsequent gel cleanup and transformation.

Figure 5. TlpA RBS Library. A RBS library of the TlpA RBS was calculated with the Red Libs algorithm. It contained 12 different RBS sequences that should exhibit different strong translation initiation rates. Green colonies have lost the repressor activity of TlpA probably due to very weak TlpA expression. Non-fluorescent colonies might have a strongly repressed gfp, because of a higher amount of TlpA.

TlpA RBS library variant selection

Single colonies were picked and inoculated to a 96 well plate and grown to a stationary phase. Continuing with the 96 well format, the samples were inoculated into a fresh 96 well culture plate and grown to OD 0.4. At this point the cultures were split and induced at 37 °C and 45 °C for 3 h. Samples were diluted in PBS and the fluorescence measured in a plate reader. The eight variants with the highest fold-change were selected for further experiments.

The best eight TlpA RBS variants were tested for fluorescence induction according to the protocol.

Figure 6. TlpA RBS Library variants on/off ratio. Distribution of obtained fluorescence expression of the library variants and their fold change. The fold changes have a high variance from 20 to 7000. This data suggests that different variants of the original RBS were compared. The new variants from the library generation yield higher fold changes than the parent variant which had a RBS with rather low translation initiation rate. The high fold change of H1 is caused by the low leakiness, not by extraordinary high expression.

Figure 7. The 96 well plate with the four technical replicates of the induction of gfp with the thermosensitive TlpA repressor. On top: 4 wells per column induced for 3 h at 37 °C, bottom: 4 wells per column induced at 45 °C. The positive control did not grow.

Triplicate measurements of the best 3 variants

Experiment was performed according to the protocol with TlpA RBS library variants H1, A9, C12 and D9. They were sequenced and compared to the calculated translation initiation rates:

The thermoswitch was now tight enough to repress the toxic protein E to enable transformant colonies to grow. It will be transformed together with a protein E RBS library containing plasmid, with the aim to find protein E RBS library variants with enough reduced translation initiation rate to survive.

Figure 8. TlpA-regulated gfp expression. left: Fluorescence/OD of the variants C12 (C), A9 (A), H1 (H) and the parent variant. right: fold changes. Different variants of the RBS of TlpA were induced and not induced in triplicates (three different colonies picked). They have a reduced leakiness compared to the parent RBS which lead to a higher on/off ratio.

To read more about each of these experiments, click on the buttons below. For a detailed protocol describing each experiment, visit Protocols.

Phase III - A: Combination of heat sensor and cell lysis

References

1. Contois, D. E. "Kinetics of bacterial growth: relationship between population density and specific growth rate of continuous cultures." Microbiology 21.1 (1959): 40-50. doi: 10.1099/00221287-21-1-40

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