Ever since I perceived the splendor of nature, I have adored the colour blue. Blue, to me, is beauty, purity and serenity. The vigour of life hides under the flickering and azure waves; The clouds hover in the crystal blue firmament; The fish joyfully rambles in the murmuring creeks; the limpid flowers bloom aside glittering lakes… When I wander in the exuberant forests, beside the brooks that giggle like wind bells, on the plains of Iowa, laughing in the smell of the new-born soil, beyond reeds that have their waist bend, under the scattering stars that wish me goodnight, I can't help ponder that the colour blue made all these possible. One thing leads to another---I found copper sulfate solution extremely fascinating when I started to study Chemistry. When solid copper sulfate dissolves in water, the color of the solution looks just like liquid sapphire. Under that delicate sapphire colour, I was devastated to learn of the danger of cupric ions to my beloved nature. This urges me to save the clear "blue" of our environment, striving me to participate in my IGEM project ---- developing a copper ion biosensor.

To detect and monitor copper ions, an accurate and sensitive copper biosensor is indispensable. Yeast biosensors have been developed for detecting heavy metal Cu2+. However, existing yeast sensors for Cu2+ rely on the CUP1 promoter, which exhibits relatively high background noise.

To achieve a biosensor with both low basal expression levels but high sensitivity, we aim to design a synthetic circuit that amplifies the signal generated by the biosensor based on the MAPK pathway of Saccharomyces cerevisiae. Firstly, the expression of α-pheromone is controlled by the promoter CUP1 in response to Cu2+, allowing fine-tuning of communication and response according to the concentration of Cu2+. Secondly, a positive feedback module is introduced to amplify the signal, increasing the sensor sensitivity and reduce the detection limit. In addition, we engineered the pheromone inducible promoter to expand its dynamic range to achieve better amplification effects. In the future, we aim to modify the CUP1 promoter to lower the basal expression level and further enhance its sensitivity.

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