Difference between revisions of "Part:BBa K3076100"

Revision as of 07:56, 14 October 2019

Coding sequence of metallothioneins (MT) gene from Corynebacterium glutamicum (CgMT)

Description

This part contains the coding sequence of Corynebacterium glutamicum metallothionein gene (CgMT). This part was synthesized into pET151 expression vector as an expression construct (BBa K3076803) and then transformed into BL21(DE) E. coli strain. The expression of this part showed a significant increase of copper absorption ability in E. coli and it suits our purpose of creating a bacterial absorbent of metal pollutants in the liquid medium.

Usage and Biology

Metallothioneins (MT) are proteins made of 61-68 amino acids with a small molecular weight. It is found in almost every known organism, from bacteria to humans. Due to this ubiquity, it minimizes the gene toxicity effect when we choose it as the ectopic expression target.

MT contains many cysteine groups which were reported to be responsible for chelating a wide range of metal ions such as cadmium, lead, copper, and mercury, etc. Nevertheless, MT from different species showed different affinity towards different metal ions.

Literature reported that MT protein from Corynebacterium glutamicum (CgMT) shows strong binding affinity towards divalent cations, such as Zn2+ and Pb2+. [1] Since our project used Cu2+ as a model for the metal pollutants, we decided to explore the plausibility of using CgMT to increase the metal accumulation ability of E. coli.

Copper removal assay

The E. coli expressing CgMT was put into copper (II) sulphate-containing medium. The concentration of copper ions in the medium was measured at different time points. API copper testing reagent was used in this assay. The reagent reacts with copper ions and forms brown colour precipitate. We used a colourimeter to measure the absorbance (440nm) of the reagent mixture and calculate the concentration of copper ion by a standard curve. (Fig. 1)

Fig. 2 This standard curve was constructed by using standard solutions of copper (II) sulphate. The absorbance (440nm) of the API reagent and standard solution mixture were measured by colourimeter. The linear equation of the standard curve was then used in other assays to convert the absorbance value into concentration.

We found that when compared with no IPTG induction control group and empty vector control group, the CgMT gene -expressing group removed significantly more copper ions inside the medium after 4 hours of incubation. (Fig. 2) This result indicates CgMT gene expression can effectively increase the copper absorption ability of E. coli and it serves the aim of our project which we tried to increase the metal pollutant removal ability of E. coli.

Fig 2. The CgMT-expressing group removed ~30% of copper ions in the medium when compared with controls removing only ~12 - 15%.

Future work

In this project, we also studied the effect of knocking out copper exporters in E. coli to see if the copper absorption in the organism will be increased. We found that by knocking out cusF gene, the KO strain significantly removed more copper in the medium when compared with the control group. (BBa K3076500) Thus, in the next step, we will combine two modifications together to see if it can further improve our metaL pollutant removal system.

References

[1] Jafarian, V., & Ghaffari, F. (2017). A unique metallothionein-engineered in Escherichia coli for biosorption of lead, zinc, and cadmium; absorption or adsorption? Microbiology, 86(1), 73–81. doi: 10.1134/s0026261717010064

Sequence and Features