Part:BBa_K2235010:Experience

Results

After confirming that the cloning worked, the plasmid was transformed into E. coli BL21(DE3) and expression was induced at multiple combinations of OD600 and IPTG concentrations. The expression of EBG, a 47 kDa protein, from one of the successful expressions (at OD600 of 0.4 and an IPTG concentration of 0.5 mM) is shown in figure 2.

Figure 2 is attached here

By carrying out the experiments above we demonstrated successful cloning and expression of both of our mucus degrading enzymes: sialidase and EBG. The next step was to secrete the enzymes, using an already existing biobrick for HylA E.coli secretion system (BBa_K1166002) from the iGEM 2017 distribution kit. Firstly, we removed the stop codon at the end of the sialidase gblock sequence using PCR and thereafter cloned sialidase without the stop codon upstream of the secretion system. The newly cloned plasmid was transformed into E.coli and expression in flask was induced with 0.5 mM IPTG. The enzyme was extracted from the medium using IMAC purification. Figure 3 shows no secretion of a protein resembling the correct size (≈ 55 kDa).

Figure 3 is attached here

Molecular Cloning

Ligation of EBG insert into iGEM backbone

We designed our endo-β-galactosidase (EBG) biobrick (BB_XXX) by modifying the sequence of professor Li (2002,). The sequence received did not contain a His6-tag, which was added to the sequence for later IMAC purification steps. The biobrick was cloned into an iGEM compatible plasmid backbone (pSB1C3). To confirm successful cloning, we double digested plasmids from five different colonies and the results are shown in figure 4. For each colony, two bands could be observed. One at ~1400 bp, corresponding to the size of EBG, and one at ~2000 bp, corresponding to the size of the plasmid backbone.

Figure 4 is attached here

Cultivation

Cloning of EBG gBlock into a pSB1C3 vector was performed with T4 Ligase. Ligation was done with ThermoFisher T4 DNA Ligase Buffer (10X) at 22 °C and the vectors were transformed into E. coli TOP10 and BL21(DE3) by heat shock. After growth overnight at 37 °C on petri dishes the results were documented.

Figure 5 is attached here

By analyzing the number of colony forming units and color of the colonies the ligation efficiency of LigA should be assessable. For the negative control, there were no clones able to grow on chloramphenicol supplemented LB medium. For the positive control, a much bigger number of clones was observed when ligation was performed.

Methods

IMAC purification

Aim: to purify the EBG samples and the control based on the containing Histag.

Procedure: The protocol was used with no changes. The colons used for the IMAC purification was one nickel colon with colon volume of 3,2 mL and three cobalt colons, each with colon volumes of 1,2 mL. The protein samples was eluted to five fractions each.

SDS-PAGE and staining

Aim: to visualize the expressed Sialidase samples and make sure the correct protein has been expressed by comparing the size to the ladder on the SDS-PAGE gel.

Procedure: The protocol for SDS-PAGE used with some modifications.

Protein samples and loading buffer were mixed at a ratio of 24 µL of protein sample and 6 µL of loading buffer in a fume hood. Loading buffer was prepared by staff at floor 2. A pre-casted gel was used (Mini-Protean TGX from BIORAD).

The samples that were run on SDS-PAGE were the IMAC purified A1-A3, B1-B2 and C1-C3. B3 and the control were not run because lack of space of the gels. These samples were run later on SDS-PAGE gel. The gels were stained using the protocol for SimplyBlue SafeStain.

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