In planetary exploration, monitoring temperature changes in the atmosphere is important for predicting weather patterns and atmospheric stability. At different heights above the planet’s surface, the temperature can vary drastically. The way that the temperature changes from hotter to cooler than back to hotter temperatures can be used to predict how weather and wind patterns will move across that region. This is helpful to know in planetary exploration because it can help predict the weather that various rovers or exploration outposts would have to deal with without relying on expensive satellite equipment. Additionally, the vertical temperature profile of a certain region of a planet’s atmosphere can be used to predict how easily a payload will fall through the atmosphere and land on the planet’s surface. [1]
On Earth, vertical temperature profiles can be obtained using electronic temperature sensors mounted on weather balloons. These balloons go up to different heights in the atmosphere and relay real-time measurements of the atmospheric temperatures. While these electronics are easy to obtain and replace if they are broken on Earth, in the context of planetary exploration, they are irreplaceable and unreliable because it would take years to send replacements if they break and are impossible to fix without also sending replacement parts along the mission. Thus, a biological temperature sensor was developed in order to address these concerns. This biological temperature sensor can be expressed in a small culture of E.coli and be easily replaced simply by maintaining a culture of that particular bacteria.
In order to build a biological temperature sensor, we used chromogenic proteins from the DNA2.0 paintbox and from the iGEM registry. Chromogenic proteins or chromoproteins are colored proteins that have been extracted from a variety of organisms such as the sea anemone, Actinia equine (Citation 2) or have been synthetically produced (as was the case for the chromoproteins from DNA2.0). Like all proteins, chromogenic proteins are sensitive to heat; however, their sensitivity to heat is coupled with a visible color loss or change. Our research has shown that different chromoproteins respond to different temperatures by either losing their color or changing to a different color. These differential responses to temperature allowed us to build a biological temperature sensor.
The basic structure of a chromogenic protein consists of a large beta barrel and a chromophore which is supported by the beta barrel. The chromophore interacts with light and gives the protein its distinctive color. Additionally, the shape of the beta barrel is supported by water molecules that interact with the side chains of the amino acids that the beta barrel comprises of. These water molecules are essential for the chromogenic protein to maintain its structure. In response to heat, our hypothesis is that these water molecules are released from the beta barrel, thus allowing the barrel to collapse and the chromophore to no longer be supported enough to produce color. (Citation 3) Because planetary exploration requires a sensor that can function in a variety of environments, we tested our chromogenic proteins in a sealed environment and in an open air environment. The following sections details those experiments.
Closed System To test our chromoproteins in a closed system that did not allow them to interact with anything in the air, we spun down 5 mL cultures of each of our chromoproteins and added 30 uL of the pellet cells to a PCR tube along with 1 uL of .165 M EDTA. We then heated these PCR tubes for 5 minutes at a starting temperature of 40 degrees Celsius, removed them from the thermal cycler, took a picture, then returned them to the thermal cycler for 5 more minutes at a temperature that was 5 degrees Celsius higher than the previous. We repeated this process until we reached 100 degrees Celsius. As can be seen from the images below, groups of chromoproteins lose their color at specific temperatures. Additionally, Cupid Pink and Donner Magenta change from pink to purple at 70 and 80 degrees respectively.
Interestingly, we also observed that if there were certain cultures that were not showing color on a particular day, the heat treatments as described above were able to cause the color we would have initially expected to appear. (Insert Picture (CP1) (make sure it’s tilted so that there are two rows of PCR tubes running horizontally), Caption: As can be seen above, the bottom row of five PCR tubes containing 30 uL of the cell pellet does not show any color. The chromoproteins in the bottom row are from DNA2.0: Virginia Violet (left), Maccabee Purple, Seraphina Pink, Tinsel Purple, and Donner Magenta.) (Insert Picture (CP2), Caption: After these previously colorless cell pellets have been treated at 55 degrees Celsius for 30 minutes, 60 degrees Celsius for 30 minutes, and 65 degrees Celsius for 30 minutes, color can be observed. This color then proceeds to disappear again as the temperature continues to increase (insert picture (CP3)). This time, the color does not return after heat treatments.