Fluorescent yeast cells microscope

New biosensors enable real-time monitoring of chemical production.

Thursday 22 Sep 16
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Michael Krogh Jensen
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DTU Biosustain
+4561 28 48 50

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Anne Wärme Lykke
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DTU Biosustain
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Living cells can be genetically modified to produce valuable chemicals. But identifying the best performing cells is often hard and tedious. Now, researchers have developed a new biosensor-system that light up the cells, when the chemical of interest is being produced.

Researchers from The Novo Nordisk Foundation Center for Biosustainability, DTU Biosustain, at Technical University of Denmark have developed a biosensor-system, which can directly measure the concentrations of bio-chemicals produced by engineered budding yeasts (Saccharomyces cerevisiae) by exposing the cells to UV light.

Today, many chemicals, plastics, and valuable food nutrients are produced from oil or rare plants. By using living cells as factories oil and plants can often be removed from the equation, making the production greener and more sustainable.

Monitoring production of chemicals in living yeast is often a time consuming endeavor because very few biosensors exist. Therefore, this system could benefit industries that produce chemicals and pharmaceuticals by means of yeast cell factories, which are otherwise not possible to screen effectively.

This extensive research has now been published in the well-renowned scientific journal Nature Chemical Biology.

The biosensor works by emitting light
"To the best of our knowledge, this constitutes the first successful direct transfer of bacterial transcriptional activators into a eukaryotic cell"
Corresponding author Michael Krogh Jensen, DTU Biosustain

As proofs-of-principle two biosensors were developed and applied for real-time monitoring of the production of cis,cis-muconic acid (CCM) and naringenin in engineered yeast cells. 

CCM is an important precursor for several valuable consumer bio-plastics, and naringenin is a very sought for flavor-molecule and antioxidant found in grape fruits. The bio-based production of these chemicals is industrially relevant, yet no effective screening options exists for yeast cells engineered to produce them.

Using advanced genetic engineering, the scientists designed and build biosensors which work like genetic switches. The input signal of the biosensors is the chemical of interest. The output signal encodes the green fluorescent protein (GFP), which emits a green glow when exposed to UV light. The link between the input and output is a regulatory protein, a transcriptional activator, which can bind the chemical, and thereby activate expression of GFP.

MJK Nature Chemical Biology new

The light intensity reveals the amount of product

Key to the new biosensors is the fine-tuned relationship between input and output controlled by transcriptional activators from bacteria. The use of transcriptional activators in a eukaryote like yeast is very interesting, since this class of regulatory proteins from bacteria have not previously been reported to work in higher organisms.

“To the best of our knowledge, this constitutes the first successful direct transfer of bacterial transcriptional activators into a eukaryotic cell,” says corresponding author Michael Krogh Jensen, Researcher and Co-Principal Investigator at the Section for Synthetic Biology Tools for Yeast at DTU Biosustain.

Additionally, the researchers showed that the light intensity from the glowing GFP was directly proportional to the produced amounts of CCM or naringenin, even at the highest measured concentrations. Hence, the light intensity directly tells about the amount of chemical produced by the cell.

Small modifications are necessary

In yeast, the transcriptional activator was engineered to bind the chemical of interest, and if this ‘match’ took place, the complex would bind to a specific DNA sequence driving the expression of GFP – leading to emission of light. The more chemical present in the cell, the more GFP would be produced, and thus, the more intense the light.

The scientists used different transcriptional activators for monitoring CCM and naringenin production, respectively. They also tested transcriptional activators for three other industrially interesting molecules. For all transcriptional activators, the team showed that a defined position of the binding site was key to making the biosensors work.

“With this design, all you in principle need as input for engineering a biosensor of choice is a transcriptional activator binding your chemical of interest, and the DNA sequence of its binding site,” Michael Krogh Jensen says.

Therefore, the scientists behind this study expect the biosensor-system to be applied for measuring accumulation of many different chemical compounds, such as fuels, food additives, and medicine, in engineered yeast cells.

 

Scientific article: Engineering prokaryotic transcriptional activators as metabolite biosensors in yeast, ML Skjoedt et al., Nature Chemical Biology (2016) doi:10.1038/nchembio.2177

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