You're probably familiar with yeast because when fermented in the dark, it converts carbohydrates into products like bread and beer. In these cases, exposure to light can hinder or even destroy this process. In a new study published in Current Biology, researchers in Georgia Tech's School of Biological Sciences have engineered the world's first strain of light-enhancing yeast that prefers bright environments.
Anthony Burnett said: "Frankly, we were shocked at how easy it was to convert yeast into phototrophs (organisms that can harness light energy). All we had to do was move a gene and they grew 2% faster in the light than in the dark. Without any fine-tuning or elaborate coaxing, it just worked."
Easily equipping yeast with such an evolutionarily important trait could have huge implications for our understanding of how this trait originated and how it can be used to study issues such as biofuel production, evolution and cellular aging.
Looking for an energy boost
The study was inspired by the group's past work studying the evolution of multicellular life. The team published their first report on the Multicellular Long-Term Evolution Experiment (MuLTEE) in Nature last year, revealing how their single-cell model organism, snowflake yeast, evolved multicellularity over 3,000 generations.
In these evolutionary experiments, a major limitation of multicellular evolution emerged: energy.
"Oxygen has a hard time diffusing deep into the tissue, so you get tissue that doesn't have the ability to gain energy." "I've been looking for ways to get around this oxygen-based energy limitation."
One way to provide energy to living organisms without the use of oxygen is through light. But from an evolutionary perspective, the ability to convert light into usable energy is complicated. For example, the molecular machinery that allows plants to use light as energy involves many genes and proteins that are difficult to synthesize and transfer to other organisms, both in the laboratory and through natural evolution.
Fortunately, plants aren't the only creatures that convert light into energy.
keep it simple
A simpler way for organisms to use light is with rhodopsin: a protein that converts light into energy without the need for additional cellular machinery.
"Rhodopsin is found throughout the tree of life and was apparently acquired by organisms acquiring genes from each other during evolution," said Autumn Peterson, the study's lead author.
This type of genetic exchange is called horizontal gene transfer and involves sharing genetic information between organisms that are not closely related. Horizontal gene transfer can cause seemingly huge evolutionary jumps in a short period of time, such as how bacteria quickly become resistant to certain antibiotics. This can happen with all genetic information, but especially with the rhodopsin protein.
"While looking for ways to transfer rhodopsin into multicellular yeast, we discovered that we could understand the horizontal transfer of rhodopsin that occurred in the past during evolution by transferring it into regular single-cell yeast."
To see if they could equip single-celled organisms with solar rhodopsin, the researchers added a rhodopsin gene synthesized by a parasitic fungus to common baker's yeast. This particular gene codes for a form of rhodopsin that is inserted into the cell's vacuoles, the part of the cell that, like mitochondria, converts the chemical gradients created by proteins like rhodopsin into energy.
Equipped with vacuolar rhodopsin, the yeast grew about 2 percent faster in the light—a huge benefit for evolution.
"Here we have a single gene and we just pulled it across environments into a lineage that had never been phototrophic before, and it works like this." "This shows that it's really easy for this system to work, at least sometimes, in a new organism."
This simplicity provides key evolutionary insights, and the researchers illustrate "how easily and why rhodopsin can spread across so many lineages."
Since vacuolar function may contribute to cellular senescence, the group also began collaborating to investigate how rhodopsin could reduce aging effects in yeast. Other researchers have begun using similar new solar-powered yeasts to study advanced biological production, which could mark a major advance in biofuel synthesis and more.
However, the team is more interested in exploring how this additional benefit affects the transition from single-cell yeast to multi-cellular organisms.
"We have this beautiful simple multicellular model system," said Burnett, referring to the long-running Multicellular Long-term Evolution Experiment (MuLTEE). "We wanted to give it photonutrients and see how that changes its evolution."