We propose

Our project aims to create a basis for biofactories that have the ability to persevere through exposure to radiation from different sources. For this, we genetically engineered Saccharomyces cerevisiae, one of the most widely used species for eukaryotic cell factories, to withstand elevated radiation levels. Even though the central goal for our engineered yeast is to cope with space radiation, it could also be used in radioactively contaminated environments or as a model system in the cosmetics industry. We set out to achieve radioresistance by introducing a bacterial tyrosinase, an enzyme involved in the conversion of tyrosine into melanin. Melanin can absorb radiation. In addition, the potential to convert the damaging radiation energy into usable metabolic energy was reported for melanin in fungi from Chernobyl (Dadachova et al., 2007). The melanin-shielded yeast could be the first step in giving yeast the ability to use alternative energy sources.

Here we present our ideas and plans for the implementation of the outcomes of our work.

Cell factories for use in space

From the onset, we envisioned the strain being used for space exploration by astronauts and companies developing space technologies. A journey to Mars takes around 7-9 months one way. However, lengthy space voyages are extremely risky because the crews are isolated for long periods without the possibility of receiving additional supplies or medicines. The best way to guarantee a sufficient supply of various resources is their on-site production. This is where synthetic biology can help because miniscule amounts, such as a few yeast cells, can be upscaled into cell factories to produce the necessary compounds. In a similar quest, AstraZeneca is investigating the potential of producing therapeutic proteins during space journeys (MaryAnn Labant, 2021). Our strategy of creating a radiation-resistant yeast could possibly be applied to the thousands of already available yeast cell factories, enabling the production of these compounds in space.

How can Earth benefit from our project?

In addition to space technology, another sector potentially interested in our radioresistant yeast could be radiobiology specialists and nuclear researchers. Our yeast could be utilized for the cleaning of radioactive contamination. Nuclear power plants generate radioactive waste and are dangerous in case of reactor meltdowns or leaks, like in the Chernobyl or Fukushima disasters. Such incidents create a need for intense cleanup projects. Research has shown that melanized fungi have the ability to proliferate in the presence of radioactivity and that they gradually engulf and destroy these contaminated particles (Zhdanova et al., 2002, Zhdanova et al., 1991). Our melanized S. cerevisiae could be used for the cleanup of radioactive contamination in case of emergency or reduction of radioactive wastes.

A novel energy source for yeast cell factories

A key challenge for our generations is to achieve a sustainable global economy. We believe that this requires searching for innovative energy resources, and we found inspiration for this from melanized fungi that seem to use melanin to convert energy from radioactive decay into metabolic energy (Dadachova et al., 2007). Although further research is needed to understand the molecular principles behind this energy transfer, we believe that creating melanin-shielded S. cerevisiae cells could be the first steps in giving the most widely used cell factory host species the ability to use novel energy sources, as in different types of radiation.

Melanized yeast as model system for cosmetics industry

In addition to the uses above, our Space Yeast can serve as a model organism in screening for the potential substances that can be used for the production of whitening cosmetic products. Usually, whitening components are tested using human-derived epidermal keratinocytes and melanocytes in multi-layered cell cultures (Jeon et al., 2021). However, working with cell culture is laborious, time-consuming, and costly. Melanin-producing yeast could be used at the initial stages of the process, preceding trials with the human cell culture. For example, yeast could be used to screen a large library of different components for their potential ability to destroy melanin or inhibit its further production. Expression of tyrosinase at the yeast cell surface would eliminate the need to use purified melanin for in vitro screening.

Safety considerations in our design

Our engineered yeast strains are genetically modified derivatives of baker’s yeast S. cerevisiae. This species belongs to the iGEM White List and is generally considered safe. We are also using the tyrosinase gene from Bacillus megaterium, a species that is also considered not harmful to humans or the environment. We aimed to engineer yeast strains that produce melanin, which is non-toxic and not harmful to the environment. To minimize the risk of distributing antibiotic resistance genes, we tried to avoid introducing the bacterial antibiotic resistance genes from plasmid vectors into our final yeast strains, where possible.

Challenges

In addition to high radiation levels, biofactories in space would face other hazards. The most notable of these is microgravity. Microgravity has been found to affect yeast cell morphology, gene expression and replicative lifetime (Fukuda et al., 2021; Nemoto et al., 2019). The effect of microgravity on the efficiency of cell factories is not yet known. A yeast gene deletion library is set to be sent to the International Space Station in order to reveal the genes that affect yeast growth under microgravity (Jessica Nimon, 2011). Results from that study could provide insights into how to improve our radioresistant yeast strains to cope with microgravity.

Another challenge is the availability of resources such as water and oxygen in space. Currently, resource supply in space is costly and risky, especially in the case of long-term missions (for example, future Mars missions). However, oxidation steps in the melanin production pathway make its synthesis under fermentation conditions impossible. Thus, alternative sources of oxygen are required. It would be desirable to investigate the possibility of co-culture our Space Yeast with cyanobacteria that can serve as a source of both oxygen and carbon (Li et al., 2017).

As we aim to make radioresistant yeast strains that can be used as the basis for other cell factories, melanin synthesis must have a minimal negative effect on the cell so that it would not affect other synthetic pathways. Further research is necessary to achieve an optimal level of melanin production to provide sufficient protection without burdening the cell metabolically.