Our device, Swallowmyces cerevisiae, bravely dives into dangerous salt water and swallow Na+, to reduce salt concentration of the water. Such microorganisms should fulfill the four criteria shown below. 1. They have to survive in high salt water. 2. They have to uptake Na+ into their cytoplasm or vacuoles. 3. By swallowing Na+, they have to reduce salt concentration of the water. 4. After use, they have to be collected quickly to trash easily. Let us introduce our project design in order.

1) Preparation of salt resistance enhancing plasmid in budding yeast

When Na+ ions are collected in S. cerevisiae cells, high concentration of Na+ might damage the cells. We have to develop tools which protect S. cerevisiae cells from salt damage. We used two devices here, “compatible solute synthesis” and “chaperon-like protein”. Did you know that a certain group of yeast is working hard in high salt condition, to provide great contribution to tables in the world? They are, Zygosaccharomyces rouxii, soy sauce-brewing yeast. They produce glycerol as compatible solute to counteract osmotic stress. We cloned from Z. rouxii ZrGPD1 (glycerol-3-phosphate dehydrogenase) and ZrFPS1 (glycerol transporter), both of which are important for the glycdrol metabolism in Z. rouxii. We tried to increase salt tolerance of our Swallowmyces cerevisiae by these two genes. Another gene we focuced on is mangrin, a small peptide from mangrove, salt resistant plants. It is a chaperon-like protein and believed that it repairs salt damaged proteins. We decided to test this gene to increase yeast salt tolerance.

2) Preparation of yeast to incorporate Na+

S. cerevisiae has Na+ transporter to remove Na+ from their cytoplasm. The main transporters includes NHA1, ENA1, ENA2, ENA4. When all of these genes were knocked out, the deletion strain shows hypersensitivity against NaCl. To produce yeast strain that uptake more Na+, using homologous recombination system, we knocked-out all of the above genes. Furthermore, we found a protein which is involved in influx of Na+ in plants. It is AtHKT1. By overexpressing AtHKT1, we expect that more Na+ will be collected by the yeast. Another candidate we found is McHKT2. McHKT2 is a Na+ transporter of Mesembryanthemum crystallinum. M. crystallinum, “ice-plant”, is a salt tolerance plant. It is salty. It is reported that McHKT2 is involved the salt compartimentalization in this plant. High concentration Na+ in cytoplasm might damage the cells. To overcome this problem, we will use the salt plants’ salt tolerance system, where Na+ in cytoplasm is sequestered into vacuole by Na+/H+ exchanger. These factors includes Na+/H+ antiporter AtNHXS1 from A. thaliana, SseNHX1, a paralog of AtNHXS1 from salt plant, and a vacuolar protein AVP1 which inclease H+ concentration in vacuole. By enhanceing Na+ influx, by preventing Na+ efflux, and by redirecting cytoplasmic Na+ into vacuoles, we aim to create yeast strain which accumulate more Na+.

3) Reduce the concentration of NaCl in the medium

The goal of our Swallowmyces cerevisiae is not the uptake of Na+. We aim to reduce salt concentration of the water by this device. By combining experimental data and mathematical modeling, we try to optimize our system, to achieve maximum desalination. What will happen when we mix our best strain into high salt-containing media?

4) Development of aggregation system

Even if our device efficiently reduce NaCl concentration in the media, we will never stop our research and development. We try to construct robust safety system for the biocontainment of our genetically modified yeast cells. For this purpose, we selected two gene, SdrG and FgBeta. SdrG is a surface component of Staphylococcus epidermidis. It is known that SdrG tightly binds to a small N-terminus domain of human fibrinogen beta. Reportedly, the binding between SdrG and fibrinogen beta is as strong as a covalent bond. If we express these proteins on the surface of yeast separately, we might see strong coupling of two yeast cells mediated by this interaction. In such a case, as one cell will display multiple handles, we might see a big ball of conjugated yeast cells, when we mix the two “hundle-desplaying” strains.

SdrGはStaphylococcus epidermidisのsurface componentです。このタンパク質はヒトのfibrinogen beta由来の25アミノ酸という短い配列に非常に強く結合すること、これが生体内へのバイオフィルムの固着に関与していること、などが示されています。これらのタンパク質を別々の酵母でsurface displayにより発現させれば、強い結合力を利用して２細胞の強い結束が実現できるのではないでしょうか？　一つの酵母が複数のハンドルを発現するとすれば、二種類の酵母をブレンドするだけで、酵母は全体で一つの大きな細胞塊になってしまうのでは？　 さて、上の1-4まで、順番にどうなったか、見てみましょう。　→　（リンク）