Improvement

Overview

Our composite part BBa_K4288005 was improved based on the existing part BBa_K174015 submitted by iGEM09_Newcastle. In 2019, group iGEM_Gaston Day School combined this part with an RBS (B0036), RFP (E1010), and a double terminator (B0014), and has shown that this promoter is sensitive to cadmium down to 0.25M (p<0.01). Today, our team further improved the cadmium sensor by optimizing the sequence of cad promoter, fusing a new signal peptide of RpmG which helps protein secret outside the cell, and combining a report protein feruloyl esterase(FAE). Therefore, the new composite part BBa_K4288005 was developed.

Figure 1. The DNA sequence difference between BBa_K4288005 and BBa_ K174015).

In order to prove the function of our new composite part cadA promoter-signal peptide of RpmG-feruloyl esterase, we transformed the part in Bacillus subtilis, induced FAE protein expression with various concentrations of cadmium nitrate, and detected the enzyme activity of FAE. The result shows that our biosensor can detect the concentration of cadmium ions within 50μg/L, which broadens the concentration range of biological detection of cadmium ions and takes a more important step towards practical application.

Introduction

As more and more human activities create accumulating impact, and as industries develop at an unprecedentedly rapid speed, huge amounts of heavy metal pollutants are released into the environment, contaminating the earth. Heavy metal ions such as Cadmium (Cd), Chromium (Cr), Lead (Pb), Mercury (Hg), and metalloids like Arsenic (As) cannot be degraded by organisms. As a result, these ions can accumulate, create chronic toxicity, and manifest bioaccumulation up the trophic level.

Humans then take them in without notice, either by directly drinking contaminated water or through eating other plants and animals that are previously exposed to heavy metals. If the heavy metals enriched in human bodies, they can cause serious symptoms such as diseases in the kidney, the lung, the liver, the bone, different types of cancers, and deficiency in embryonic neurodevelopment.

Therefore, it is urgent to develop a fast and convenient tool for the detection of heavy metals. Our project designed a cadmium biosensor to detect the heavy metal cadmium.

Construction of Cadmium-biosensor plasmid

We utilize the operon CadA to expresses reporter feruloyl esterase in Bacillus subtilis and the plasmid pHY300PLK-PcadA-Biosensor was handed over to GenScript for synthesis. The synthesis report is as follows.

Functional test

We tested the ability of cadmium-biosensor for detecting the heavy metal cadmium. The plasmid pHY300PLK-PcadA-Biosensor was transformed into Bacillus subtilis(Fig2.) and tested while in different concentration of cadmium nitrate. The enzyme activity of feruloyl esterase(FAE) is to reflect the cadmium concentration after the induction of cadmium ions. Under different cadmium concentration scales, the enzyme activity of FAE is shown in the figure 3.

Figure 2. Plate of the transformants of pHY300PLK-PcadA-Biosensor/Bacillus subtilis.

Figure 3. The activity of Ferulyol Esterase in different concentaiton of Cd. The data were collected in 0.5h.

As shown in the graph, the more the cadmium is, the more enzyme activity it has. While, when the concentration of cadmium reaches 50μg/L, its enzyme activity value tends to be stable at about 0.35. We speculate that when the cadmium ion concentration exceeds 50μg/L, the host Bacillus subtilis cannot express the protein normally, so the measured enzyme activity value is flat. This indicates that our current cadmium biosensor is suitable for the detection of samples with cadmium ion concentration in the range of 0-50μg/L.